You will get the most battery life out of surfing the web and word processing on low brightness, I used to get about 3.5-4 hours life for those tasks. Then I could easily watch a movie on full brightness, and then some, so call it 2.5 hours. Finally, for gaming.... well I dont have a gaming laptop, but I have gamed on mine, and although I get a little bit worse battery life in windows (about 30 minutes less for any given task) I was still able to game for over an hour before the battery started yelling at me. 1.5 hours or thereabouts I suppose.
That is on my machine, other systems will vary either up or down. But that at least gives you an idea of the battery draining-ness of different tasks. Also, A gaming laptop (also known as a DTR) will get, if you are lucky, 1 hour of gaming time unplugged. But in my opinion the battery is there either as ballast, or so you can keep gaming after the power goes out.