The previous answer is, of course, entirely correct. However, a couple of tips learned from experience... If you charge your Kindle from your computer, it will take 5 , maybe 6 hours, depending on discharge. Problem is, if you plug it into the computer and leave it as soon as your computer goes into sleep mode or otherwise turns off, the charging will stop as well.
Secondly, if you're operating from Wi-fi, the battery will have almost half the charge life as when the device is not receiving via Wi-fi. So... when your not using your reader, turn it off or disable the Wi-fi mode to extend the battery charge life...