If I had to charge my iPad with the case off, the case would never be on. I seem to have it on the charger more than off.
I think my iPad2 had a much longer battery life, but Air 2 is too new (3 days) to be certain. I use mine in the car for work, and GPS does use a lot of battery juice, but after two days use in the car, I think the Air 2 battery drains twice as fast as my old one did. My point being, I need the case on, and I need it plugged in whenever possible too.