Mains should be about 2-3 hours, USB 4-5 hours roughly. Try charging it on your PC without AI Charger. If your motherboard doesn't support AI Charger (all Asus motherboards are not supported), it might be slowing the charging down, instead of speeding it up. Do you know someone with an iDevice that would let you try to charge it up with your iPad charger and your PC, to see if they have any problems charging. If they don't then, worst case it's probably a bad battery.
And, one more thing, have you tried letting the iPad discharge completely until it shuts off, and then charge it up fully, this is supposed to calibrate the battery indicator. I could simply be that it's really charged fully up, but the indicator is wrong, never showing a full charge on it. The mains charger getting hot worries me. Points to bad charger or bad battery.