Using the iPad wall charger, the nominal charge time from 10 to 100 percent would be 5-6hours or so. It won't overcharge if left for more than 6 hours since there is a sensing system which will automatically limit the charging current to a trickle charge rate.
I like to recharge my Pad battery when it is between 20-30 percent, usually, and once in a while take it down to around 10 percent before charging.
Lithium polymer batteries do not have the memory issues associated with old nickel cadmium batteries.
Charge it whenever you want or feel the need to, it will not affect the battery.
All modern day electronics have circuitry built in to prevent over charging. Charge it for as long as you want, it won't hurt the battery.
Here is Apple's official statement about the battery: