Is it a must to have a frequency ear training to accomplish a good mix? How long will it take for one to at least internalize the one octave increment? Thank you.

There are several things you can do to train your ear. One is to listen to sinewaves of various frequencies on a regular basis and yes you will start to be able to identify them. (BTW where this comes in real handy is when you are doing a live mix and suddenly the PA or foldback starts taking off a bit. This sort of practice will help you nail that frequency first up and kill it pretty fast)
 
Another approach which is better in my opinion is to listen to pink noise through a graphic equaliser type plug-in and make it a 10 band to make it a bit easier. Slowly raise and lower each band and listen to what is happening.
 
A better alternative again to that is to listen to music that is very well mixed and balanced (like a Steely Dan track or just any great well mixed commercial track) through the same said 10 band EQ and gently raise and lower the bands and get a feel for what you are hearing. After a while you will start to identify the freq bands you are manipulating.
 
It just saves a bit of time in terms of identifying an area of a track for example that may be excessive or lacking. But you can still learn this type of stuff through just experience. Like you have recorded an acoustic guitar track and it may be boomy say. The problem may be around 250 Hz but you pull down 500 Hz instead and nothing much changes so you try 250 instead and viola there is a significant change for the better so by doing that you have just learned what excessive 250 Hz sounds like and it tends to stick. I am sure many great practical mix engineers have learned it all this way and it is probably the best way in the end.
 
One approach might be to create a series of tests for yourself by taking that well mixed music and boost one band by say 6dB. Save that and put it away for a few weeks and then pull it out and test yourself by listening to it and seeing if you can identify which band it was and then try pulling it down. Have the track flat or with no boost easily available to you for a quick reference too. You could make notes as to what band it was and not look at first but then after check your answer.
 
It is a little harder to hear a certain area of the frequency spectrum that is lacking and know what band you need to boost to make it normal again. But if you start with cutting excessive areas first you will find you start to identify the dip areas after a while too.

This website offers free "Ear" Training...

I haven't used it so can't vouch for it..but it's free...

http://www.v-plugs.com/mr_soundman

@ Jeff Evans, thanks for taking your time in answering my query. Little by little i want master this part. I asked this question for the reason that i downloaded the quiztones app. for iphone but i dont know where it leads me whether its useful or just wasting my time, now i know. i still may not have enough training that i cannot feel/ hear the effect when i try to mix. Thank you again!

@ Cclarry, yea i have that one and i like using it, every time you play with it it gives you a score that if you reach a certain point it gives you a discount to buy the full version.

I went through a book and CD training for frequency identification, etc. and for me, those "training" methods never worked. What I did to train my ears is to isolate a single instrument (i.e. guitar, keyboard, bass etc.), then grab a good parametric EQ that can set a very narrow Q and a decent spectrum analyzer. While the instrument is played use the EQ to either cut or boost at a tight (high) Q while sweeping the frequency spectrum. This will take many sessions, and in my case, where I grew up with a stereo EQ with a built-in spectrum analyzer, eventually, you can begin to "hear" the approximate frequencies of the signal. 
 
In the 80's I was obsessed with lighting the top LED's of 16khz while dubbing cassettes. If I could not get that top LED to light on the copied cassette but the original would, the recording wasn't up to snuff for me, and I spent hours and hours with azimuth and bias adjustments, as well as trying many different brands of cassettes with different metals in the tape.
 
Eventually, i just started hearing what I needed to. Now, I don't need a spectrum analyzer, and don't need to use EQ's that show Q, Frequency, etc.
 
The bottom line is, if you know what sounds right, you don't always need to identify frequencies exectly, unless you're going to be working in audio research, for example.

You also need to train your ears to your room and Tune your room to the flattest you can possibly get it. If you cannot hear the true sound of the mix, the mixes will not translate onto other sound systems. Then you'll fall into just having it sound good in the room your mixed it in and it sounding like "crap" on other sound systems outside your studio.
 
Cj

The quiztones app is decent. I use that and the Critical Listening Skills book. They have helped me a bunch, especially with drums.

Frequency ear training can help a bit, but not as much as having your room optimally set out, decent monitors and appropriately placed sound absorbent panels/bass trapping.

Also this thread would be better suited in this... http://forum.cakewalk.com/Techniques-f9.aspx section of the forums ;) 

Is there a way i can move this thread to techniques. I see some forum where they can move it to other topic or only a moderator can do it? If not i'll just post a new thread in there. Thanks.