I have had my macbook air for 2 and 1/2 years now. Is this a normal time for the battery to be needing a replacement?
It's supposed to last for 1,000 charge cycles. How long that is in years depends on how you use your MacBook Air (but it should almost certainly be more than 2½ years). You can check your current value by clicking the Apple menu, selecting "About This Mac", clicking the "System Report..." button, selecting Power and looking at the "Cycle Count" statistic.