Its a electronic device, and because of that, it could fail tomorrow. On saying that, a lot of people around here are posting on this forum with a 6/7/8 yr old Macs.
To ask how long it will last is like asking how big is a Hole or how long a piece of string is.
As Nick pointed out, a battery is far cheaper than a new Mac, and even if it lasts another year and you need to buy again, I think you would of made your $ back.
Buy a battery, and then follow Apples
Battery Calibration guide to get the most out of it, so it could last another 4 yrs and more.