If the book has the lending option turned on by the publisher, the official loan period is 14 days, however that is from the time the recipient accepts it(they don't have to accept immediately), Amazon did reach an agreement in 2011 though to let libraries check out/loan their E-books in a similar manner to how they were doing with other ebooks(most notably ones for the Nook), most of the libraries ebook lending programs are run by a service called Overdrive, haven't personally used it yet but I've read it works well.
Personally I have a kindle.....plus a kindle fire, plus the app on the phone, and the computer. Most of the books i read on it are cheap or free ones, I've read several hundred through the kindle devices over the last 2 years and could probably count on 2 hands how many i paid more than 5 bucks for, most were either free or 1.99 and under. The bestsellers that I want I either rent or borrow