http://www.wired.com/gadgetlab/2013/09/laptop-battery/
This article is for laptop batteries, but I assume it would be the same for the Kindle and smartphones/tablets since they all use Lithium-ion polymer batteries.
Stop charge at 80%. Charge at 40%. That's optimal, but doesn't mean people will do that since it's kind of annoying. Also, the Kindle lasts so long those extra battery charges might not even matter.