simple question for you all.
how long should a 15inch macbook pro's battery last?

I won't know for sure until tomorrow when I get mine, but word on the net says around 2.5 hours. However if you turn the backlight down, turn off the bluetooth and turn off your sound card (mute) you could get better.

3-4 hours is about average

i actually meant how long do they last untill you need a new one.. just got a macbook pro yesterday and was just wandering. by the way amazing machine love it!!!!

It's hard to say but anywhere from 300-500 cycles before most people will start to see a drop in the batteries life. Its pretty hard to give anything more then that as it will vary alot between systems depeding on how you use them.
If you want a date figure.... very hard to say but 2 years give or take

Really? Gosh I hope so. I'm contemplating buying an extra one tomorrow.

Most Li-Ion / Li-Polymer batteries nowadays can average 500 - 750 [full] charge cycles. So, 1.5 - 3 years of use essentially.

im also thinking of buying a spare to keep in my computer bag