Yes, 2-3 hours on a working day is fine. I'll add an additional restriction. Don't study for more than about 2 hours in one sitting. So do 2h sessions on weekdays, and (if you desire) you can do a couple of 2 hours sessions (with a substantial break in between) on days you're not working.
Re: overall length of study time, most people take around 3 to 4 months, but 5-6 months is also quite common. It depends upon the level of score improvement you want to achieve, as well as how much and how effectively you're studying, so there's no way to say "oh, you need X months." Set up a good plan for yourself, start studying, check your progress periodically, adjust your study accordingly, and so on. When your practice scores start getting close to the range you want on the real test, then you know you're getting close to being ready.
Note: the test is changing in June of 2012, so if you're planning to take in April, make sure you leave yourself plenty of time in case your study schedule gets delayed or you want to take the test a second time (in case you don't like your first score).
You may want to read these articles:
http://www.manhattangmat.com/blog/index ... tudy-plan/http://www.manhattangmat.com/blog/index ... our-study/http://www.manhattangmat.com/blog/index ... ice-tests/http://www.manhattangmat.com/articles/a ... roblem.cfmhttp://www.manhattangmat.com/blog/index ... anagement/