To build on @alexandru's and @ Randolph's answers, much of it depends on your individual usage.
I have a pre-unibody MPB 15", which I've had for three years. In that time, I've gone through 3 batteries (about one per year); fortunately for me, Apple has replaced the batter twice at no cost to me (other than my Applecare warranty).
After speaking to Apple 'geniuses' a couple of times, and once more two nights ago, you need to fully drain the battery at least once a month (run it from a full charge to 0% (where it turns completely off and goes into hibernation), then charge it back up to 100% without unplugging the power cord. If you can't 'cycle' the battery in that way, as they call it, I was also told to try and keep the battery between 20% and 70% charge at all times. Setting up an event in iCal to remind you is also helpful.
Again, your usage will affect battery life (duration of charge) and watching videos, listening to music, extensive wireless communication (web browsing or transferring files over the network), gaming or anything CPU intensive or spins the hard disc or optical drive often will drain the batter much faster than, say, writing a paper in Word.
If you're experiencing very bad battery life early in the machine's life, take it into an Apple store and explain the problem. In both my cases, the Genius I spoke with replaced the battery for free then gave me tips on how to extend the new battery's life (which I'm passing on here).