Have not charged from fully drained state, but did 48% in just over 3.5 hours, suggest about 8 hours to charge from 0% (stand by mode almost entire time).
Actually boggle my mind, all that battery power. 25 W·h(!).
...Considering the charger is 2.1Amp, for 25W-h, suggests over 10 hours to charge from 0%. (musing)
The battery is 42.5 watt-hours.
The charger is rated for 2.1A, 5.1V. 2.1*5.1=10.71. So, in THEORY, you get 10.71 watts but with the voltage loss on that flimsy wire, you'll never see more than the 10watts total power the charger is marketed as (4.76 volts at 2.1A, and technically it's not a charger, it's a power supply, the actual charger is in the iPad, but still...). So we'll run with Apple's 10 watt figure, also because it's a nice number.
That means that theoretically, assuming 100% efficient charging, the battery COULD charge in 4 hours, 15 minutes (4.25 hours). Nothing is 100% efficient, and the last 10% or so of a lithium ion battery charge must be done very slowly. Still, with the device off, you should get at least mostly charged in 5-6 hours in paper. If you're using it at all, this goes down.
Now, if you use a smartphone charger... 1A at 5V is 5 watts. Theoretical minimum charge time would be 8 hours, 30 minutes. Again, assuming non-existant 100% efficiency. Also again, the device is OFF in this assumption and any use is even more likely to prevent the iPad from charging at all, and making it more likely to drain (just more slowly) while connected.
Finally, with a computer USB port or standard phone charger... 0.5A at 5V is 2.5 watts. Theoretical minimum charge time becomes 17 hours (100% efficient charging again, not realistic). In reality, the chances of this charging the device are slim, but at least the battery will drain a bit slower...
