I don't know how long it takes to charge, since I usually charge mine overnight. However, you can leave it plugged in as long as you like. There is built in over-charge protection, so the battery will not be damaged by just leaving it plugged in. Mine gets plugged in when I go to bed, but it just stays that way until I next use it, which during a busy work-week is sometimes a couple of days.
I seem to remember reading on some of the nookcolor bumpf that it takes approx 3 hours to fully charge and as long as you are using the official cable there is overcharge protection so leaving it longer should have no adverse effects.