Amazon:

Eligible Kindle books can be loaned once for a period of 14 days.

I don’t know about you, but I rarely finish a book in just 14 days — I am a slow reader and have way too many other things going on. One would think that Amazon (by the very nature of their business) would have a damned good idea of just how fast people are reading books — 14 days seems a little bit faster than I imagine most read.