Depends a lot on the battery capacity and the charger output capability. The available stock batteries are 1400ma and 2750ma capcity. The USB spec limits current to 500ma therefore a lot of chargers are only capable of 500ma output. The HTC stock charger included with the TB has a 1000ma output capability.
So do the math...if you have the 2750ma battery and the 1000ma charger the best you could hope for with 100% efficiency is 2.75hours. Since nothing is 100% efficient you are looking at 3 to 4 hours for this example....or double that if your using a 500ma charger.