These sorts of batteries do NOT require an initial charge like that... they actually come already partially charged (40% nominally) to ensure they don't arrive too discharged (which could damage them) and if the OS will let you you can simply use the device straight away.
If you want, just turn the thing on and use it. It's obviously better if you charge it up a while so you can use the thing for longer, but you need charge only until it stops indicating that it is charging (the lightning bolt symbol changes to a plug symbol, shortly after you hit 100%).
Attempting to charge for longer than that (e.g. 12h) with the regular charger is completely pointless since the device STOPS charging once the battery is full. You'd just be sitting there with it on the charger but accomplishing nothing, like a doofus... :-)
With the screen off, the regular charger should take about 3.5h for a full charge, longer if you're actively using the tablet. The rapid charger will take about 3.2h maximum, whether you're using it or not. Charging from USB can take a good 12h though, even with the screen off, and if you're actively using it it may actually be discharging the whole time (though slower than it would be if it weren't plugged in).
