I do about 2 hours of meditation a day. This is useful because it builds a huge capacity for emotional regulation and lets me stand in situations I once would have run from or otherwise sought to avoid, and and I can still clearly see what's going on in these situations. Exercise has its merits (I also walk about 2 hours a day) but they are very different.
As Suzuki Roshi said "Sit Zazen. To live life without sitting Zazen is like winding a watch without setting it: It will run, but it won't tell you the correct time."
I also sit for 2 hours a day, and I see the same benefits. I wouldn't normally post a "me too" message, but I don't see too many comments about meditating 2 hours a day.
I started out reading Wake Up To Your Life, built up to doing about an hour a day, went on a retreat[1] run by the author of that book, from which I saw huge benefits, started meditating for really long stretches in order to hammer home the skills I'd learned there, then settled down to an hour after rising and an hour before bed. (Plus any time it's useful during the day.)
If you do vipassana via http://www.dhamma.org/ the ideal daily sitting schedule in day to day life is an hour in the morning and an hour in the evening. If you make it part of your routine, it can be somewhat easy to do. It's actually trivial to be on the 2 hr/day schedule after finishing a 10 day sitting since 2 hours is nothing at that point and you remember exactly what you're doing, but in my experience it's difficult to start up again with regular 2 hr/day sittings once you stop. My routine for a bunch of years was to do the 2 hour sittings for a while, but eventually fall off until going to another 10 day sitting.
Even though 2 hours seems like a lot, because of what it is (focusing the mind, learning to not overreact to external stimuli, stability of the mind), it makes people able to be very highly productive throughout the day.
That said, 2 hours is a pretty extremeTM amount of time to meditate every day. Fifteen minutes, give or take, will still go a long way. (Fifteen minutes in the morning AND before bed is even better, though, and who doesn't spend half an hour a day just farting around?)
Yes, I don't mean to imply that two hours a day is a necessary threshold. I brought that up in response to the statement about exercise, to highlight how valuable it is to me.
As Suzuki Roshi said "Sit Zazen. To live life without sitting Zazen is like winding a watch without setting it: It will run, but it won't tell you the correct time."