depends on several factors. how old is the battery, how often do you end up draining the battery, do you have the a/c plugged in and your battery plugged in all the time. you should notice that your battery will drain a hell of alot faster if you're watching DVDs. If you just typing up a paper should last longer as the processor load would be reduced.
currenlty, my laptop battery lasts about 5 mins... well 2 minutes then starts beeping at me
take care
sam