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

All the terms of coverage are spelled out clearly here:

AppleCare Products - Mac - Apple

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.

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

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