It can take anywhere from three to five hours to charge an iPod Nano if it is low on battery life and needs to be fully recharged. The Nano does seem to charge faster if it is plugged in an iHome or home outlet instead of your computer, however if you do use your computer, make sure that your computer does not go into sleep mode or the Nano may not start charging again until the computer is back in use. It will also charge faster if you plug it directly into your computer instead of from a USB hub.
Resource: Support Apple