I think 45 minutes (up to an hour maximum) is good for an album.  Anything less feels like an EP or a bit of a ripoff and anything more is just too much now (although I remember being very happy that the Cure's Disintegration was 72 minutes at the time).
 
Remember when two albums fit perfectly on a 90 minute tape?  Then we had to start getting 100 minute tapes because some albums began to go a little over 45 minutes and, finally, we ended up with 120 minute tapes to make sure things fit!
 
Now, with MP3 players, none of that matters - heh.  That said, I'm with Steve in saying that I prefer a band to produce twice as many albums instead of a single long one.  It kind of keeps you wanting more and you don't have to wait as long to get another "fix" - lol!

Off the top of my head, the only double (studio) albums that really work for me are Electric Ladyland and Physical Graffiti. The Wall also works, but that's a whole different kind of journey.
 
Even some of my all time favorites like NIN The Fragile I find too long. Not that I don't like the songs, but running at over 100 minutes, there's just too much to ingest at once. Not to mention that beyond 70-ish minutes, you practically need to schedule time for those things in your agenda.
 

In the days of vinyl you were pretty much stuck to about 20 mins a side. Thats what done for Yes on Tales from Topographic Oceans, stretching each of the four pieces to fit the one side of an album, while its is one of my all time favorites IMHO two of the pieces were far too long. Artistically it would make sense for an album to be as long as it needs to be, not restrained by some norm as to its length. But I guess many would feel short changed if they go a 45 miniute album these days.
 
I still like the idea of "concept albums" or at least albums were there is a coherent relationship between the songs and where the play order has some meaning.
 
Deciding the play order even for albums of unrelated songs used to be an art form and it can be quite startling when the familiar order is disturbed.

Glyn Barnes
Deciding the play order even for albums of unrelated songs used to be an art form and it can be quite startling when the familiar order is disturbed.

Definitely!  What would Zep's Heartbreaker be without Living Loving Maid following it?  Or the Car's Moving In Stereo without All Mixed Up immediately after?  There are a bunch of examples that sound jarring when played separately.
 
Obviously, this is even more apparent with concept albums.

The best albums are like the best collections of short stories or books of poetry. Individual items can stand on their own but the juxtaposition can express a theme and unity that makes the whole greater than the parts. The worst albums are like the way cable TV bundles a lot of junk with a few good channels or the way mortgage derivatives were put together to sell the package at more than they could get for the good stuff by itself. Too many albums in the past were of that variety, and it is little wonder that given the chance to unbundle the bad ones to extract a couple of decent songs, the public has lost interest in the concept of an artistic collection.
 
Nothing new here really. When I was young records were two sides = two songs, one of which was almost always dreck (can you say flip side?), and not always by the same guy who did the side that sold the record. Of course in those days it was unusual to see two rock records by the same band or artist, and the one hit wonder was a reality of the business. Albums played a bigger role than most people think in making the artist as important as the song. With web distribution and every guy who can manage an upload declaring himself a star, it is quite possible that a single hit will define a musical career again.

slartabartfast
With web distribution and every guy [or girl] who can manage an upload declaring himself a star, it is quite possible that a single hit will define a musical career again.

Like that turd "Friday" by Rebecca Black that has over 60 million(!) views on YouBoob...

I'm more or less on the same page as Strummy here: many of my early double LP purchases were live offerings:
 
Made In Japan
The Song Remains the Same
The Space Ritual
Frampton Comes Alive
Exit Stage Left
Live & Dangerous
Uriah Heep Live
 
But for every one of those, we also had:
 
Focus 3
Physical Graffiti
The Wall
Moonflower
Tales from Topographic Oceans
The Lamb Lies Down on Broadway
You Are What You Is
 
Then there are hybrids of live/studio like Wheels of Fire, Sheik Yerbouti, Ummagumma
 
I guess were just damned fortunate to grow up when we did

Bristol_Jonesey
I guess were just damned fortunate to grow up when we did

We're supposed to grow up?

Singles alone just don't work for me. I like something a bit more substantial and cohesive if possible. By today's standards, a EP would be my favorite option. Maybe 4 to 6 songs, 30-40 minutes...
 
To me that's the ideal format.
 
The industry can go to heck, along with their target audience and their 10 second attention span and their 15 minutes of fame. People who do music for the right reasons couldn't really count on that system anymore, anyway. 

Rain
... 
These days, however, albums have become so incredibly long (while our attention span was taking a dramatic dive) that I rarely listen to recent albums.
...

I guess that you will never listen to a Beethoven or Stravinsky, or Tchaikovsky, if your attention span is stuck on the rock mentality of 3.4 minutes (that's e or 4 minutes) per song, that MUST have lyrics telling you that this or that happened, or is important so you know better!
 
That's a top of the pops mentality for a commercial society ... the veritable "Blade Runner" that you and I will be fighting against in the future, and we must stand against that complacency.
 
It's like saying that you are getting too old and making love now is ... 3 to 4 minutes, over and out! And then wonder why things didn't go right! And life is getting boring ...
 
It's not even about the album! It's about the people that push this stuff that are trying to make you feel guilty for not buying it. It has been like that for 40 years, when music companies were complaining about all these small bands stealing sales from the larger groups and artists ... but nothing was said that those same bands and artists were putting out total ****, as an excuse for more **** and call it art. Sort of a Jean Genet just came alive and is delivering us "Our  Lady of Flowers" in one nice neat little package ... and you are supposed to like it. Well, the language is pretty and the descriptions are insane ... but in the end, it's sick and sad and pathetic!