Okay, I just got this app that tells me in detail about my battery and the 496 cycles worried me. Must be from all that Minecraft...!
How soon should I have to replace my battery?
You're averaging about a cycle a day, which isn't excessive. Run on battery when you need to and plug it in when you can. Just make sure you don't run plugged in all the time. I prefer iStat Pro, since it tracks much more than battery data, and doesn't suffer from inaccuracy problems like coconutBattery has. This should answer most, if not all, of your battery questions: