If I buy apple care for my macbook air, how long is the term of coverage ?

Oddly enough, Apple's webpage on AppleCare + does NOT explain the length of coverage. It's for 3 years, so an additional 2 years of warranty.

From BestBuy website:

AppleCare+ for Mac extends your coverage to three years from your AppleCare+ purchase date and adds up to two incidents of accidental damage protection every 12 months, each subject to a service fee of $99 for screen damage or external enclosure damage, or $299 for other damage, plus applicable tax.

All the terms of coverage are spelled out clearly here:

AppleCare Products - Mac - Apple

This is true. The page Woodmeister links to does NOT say what the duration of the plan is.

Click on the link on that page to see the full terms of Applecare+ coverage: