Firstly I would not be too worried about how long a mix takes unless you are charging for it or they have a limited budget then you have to be a little savvy for sure.  If it’s for love and enjoyment then some mixes may take 10 hours or more to mix.  It may just be the manner of the beast.  Don't sweat it so much just spend the time that is needed.
 
When doing it within a budget then I have a plan.  Suppose the budget only allowed for 10 hours to mix one song and this is often about right too.
 
I split a total mix into 3 sections.
 
1    General stuff. (eg 4 Hrs here = 240 minutes = 6.5 mins a track in your case Chuck)
 
Routing, EQ's, dynamics, panning and rough levels eg an overall balance. So lets say 3 1/3 hrs for this eg 200 minutes for this section alone.  If you have 37 tracks then that is only about 6 mins per track to get a sound! That will sort you out.  The reality is if you cannot get a sound happening in say 6 mins then you may need to practice this a lot more.
 
Note: After many years of mixing I can usually do this first part quite easily.
 
2    Reverbs delays and effects. (3 Hrs here)

 
This can take quite a while. If you get ahead with this section then go back and keep on working with section 1.  3 Hours sounds a lot but spend the time getting all the important reverbs happening and sounding sweet.  This also may include drum reverbs, snare reverbs and solo parts reverbs
 
3    Making moves. (3 Hrs here too)

 
Fine tuning levels and balances. This is all about automation and what moves either you or the computer (or both) is going to do in a mix.  As Chuck has pointed out this can swallow up some time.
 
 
Notice like Danny said this does NOT include a lot of mix prep beforehand such as level checking of every track (if you are sent a multitrack to mix) cleaning up stuff, editing, setting up, grouping and colouring tracks and putting stuff into folders. So if I get a multi that needs all this to be done, I quote on it first and treat it as a separate thing.
 
This mix prep is really important for sure. I was asked to mix a track and offered to be paid well too.  I supplied very detailed instructions on what mix prep I wanted done beforehand. But the client had done such a poor job of the mix prep I had to decline in the end and send it back. They needed to do about another 20 hours work on it before they sent it to me! eg there were tons of guitar tracks, all takes, no comping done, a real mess. I had no idea what to use. I should not have to work that out either.
 
If the mix prep is good though this plan can and usually works out for me.  It does stop you from spending 5 hours on a kick sound!    You can't do this under pressure. You have got to get it quick and keep on moving on. I have a clock/stopwatch handy too when I am into section one a lot.
 
Many good engineers get it happening within a time frame like this.

I'm just a beginner and do this as a hobby so I'm just learning what it takes to get a good mix.  I think there are different expectations when it comes to mixing for fun versus mixing for hire.  So here's my inexperienced thoughts.
 
Mixing for fun is easy, take as long as you want and experiment so you can learn.  Use this time to try different workflows, develop templates and find ways to increase your efficiency
 
Mixing for hire you need to know how you spend your time on a project.  How much of your time is spent preparing for the project, setting up the project, importing tracks, naming tracks, grouping tracks, cleaning up tracks and comping tracks?  How much of those tasks could have, or should have, been performed by the customer?  How much time do you spend setting up effects?  Can you use templates that include effect busses to reduce that time?  How much time do you spend mixing each track?
 
Mixing for hire you have to consider both your and your customer's expectations.  You need to know what the customer expects you to do and the customer needs to know what tasks your price includes.  As others have stated project setup, track cleanup, mixing and production are different tasks that require different mindsets and should be itemized in your bill; even if you quote an inclusive price to the customer.  It is important that your customer understands what they are buying with their money.
 
If a customer is inexperienced the customer may not know what you expect from them so it is up to you to educate your customer about what your requirements are.  If you expect tracks to be named, grouped and trimmed those requirements need to be in the quote.  Do you want track names like guitar 1 and guitar 2 or strummed acoustic guitar and fingerpicked acoustic guitar.
 
Many studios have an online presence listing project and track preparation preferences, sample project, downloadable templates and other examples or tools to help the customer save you time.
 
To answer your question more specifically, yes it is okay to spend 10 hours on a mix if both you and your customer believe the time was well spent.

I think 10 hours for a 36-track Rock project is quite impressive!
From my point of view, mixing guitars is as hard as voice, because

• the mix has a great impact to the overall tone of the guitar
• distorted guitars mask a great frequency range from bass to voice and above
• the listener (and the guitarist!) is very biased about how a guitar should sound...

If cleaning up the tracks, fixing stuff etc. is within this 10 hours, it is really fast. OK, some really expensive engineers will do it probably in 3-4 hours, but if there rate is two/three times higher, where is the point from an economic view?

vdd
I think 10 hours for a 36-track Rock project is quite impressive!

If cleaning up the tracks, fixing stuff etc. is within this 10 hours, it is really fast. OK, some really expensive engineers will do it probably in 3-4 hours, but if there rate is two/three times higher, where is the point from an economic view?

See that's the thing, the time is good if you don't run a business. 10 hours on one song means you just burned a day on one thing.

You mention economics. When you pay real guys to do this, there shouldn't be editing. I don't edit like Chuck mentioned because there is no need. If you have s guy you are paying top dollar he shouldn't have to be fixing anything if his job is the mix engineer. Do it yourself guys, need to practice better recording habits so that editing is limited.

Melodyne on an entire track isn't standard procedure. It's custom manipulation. It should never just come with the territory. Again though, even doing this stuff for yourself for your own personal material, cutting out extreme editing and getting the parts right can save major hours.

That should leave you with 4-8 hours worth of mixing. 8 hours being the extreme case where you may have encountered a few issues. 8 or more hours other than for extreme cases often means you probably need to tune your listening environment.

-Danny

When I was for hire, I'd tell customers to set aside 25- 50% of the time spent recording to allow for mixing.
I also agree with many points from Danny's post....well actually all.   I record as such that clean up afterwards is typically minimal.  Clean as you go or work on recording techniques. 
Lastly, mainly coming from the days of bouncing tracks, make some decisions as you go along.  Record FX...don't be bashful.  Nowadays it seems you don't even record guitar tones until it's mix time.  How can you expressively play, without knowing how your sound interacts with the other instruments/tracks?
 
Oh... on the home brew, personal stuff... I mix as I record as I write.  Maybe much different from other folks, but when I'm done with the actual recording, the lion's share of mixing is probably already done too.  Especially with automation so easy and inserting plugs and FX.  The studio has become as much an instrument as anything, so you gotta "play it" too as you progress along.
 
EDIT...
If you're one of those guys that spends 8 hrs on just getting the snare sound...BOTH while recording and then again when mixing...  you've got some short-cuts to learn.

mixmkr
Nowadays it seems you don't even record guitar tones until it's mix time.  How can you expressively play, without knowing how your sound interacts with the other instruments/tracks?
 

This here is so true!. You're so spot on with this...great point! It's more of a curse than a blessing sometimes. The more options we have with non-destructive stuff, the more time consuming it can sometimes become. Now you're auditioning guitar sounds for hours. Lol! I do like DI tracks as a safety net, but I do prefer a client coming in with an etched in stone, core guitar tone that gets recorded physically.

Again, this is something people have to pay for. When someone comes to the studio unprepared or they wish to record the guitars after auditioning all the sounds in the VST's I have or the tones in my AXE Fx, time is money. I recommend always going in with a plan and try to rehearse the plan before the studio. Just about everyone has a little studio in their house. The practice you do there can save hundreds of dollars and time. :)