According to iLounge, the battery in iPad Air is 32.4-Watt-hours compared to the 4th gen's 42.5-Watt-hours, which will allow the device to charge faster and reduce the charging time to closer to 4 hours. If this turns out to be accurate I'll be very happy.
My iPad 3 is a dog when it comes to charging.
http://www.ilounge.com/index.php/news/comments/apple-quietly-changes-ipad-air-retina-mini-batteries/
I think that's a bit too long
The ipad 2 battery 25 Watt/Hr unit - Full Charging time was approx a little more than 4 hours or around 260 mins ( using a 10W charger)
The ipad 3 battery 42.5 Watt/ Hr unit - Full Charging time was approx around 6.5 hours or 390 mins (using a 10W charger)
The ipad 4 battery 42.5 Watt/Hr Unit - Full charging time was approx close to 5 hours or around 285 minutes (using a 12W charger)
So basically the cycle should be like this:
ipad 2 -> ipad 3 ( 70 % bigger battery)-> ipad 4 ( same battery but 30% faster charger)
If you put those figures in logical perspective:
ipad 4 -> ipad 5 (air) ( 23% smaller battery but same speed of charging)
so therefore: ipad air should take approximately 3.7 hours or 220 minutes to charge. It could be even less ( but I am putting some leverage for discrepancies and efficiency).
I definitely think that ipad air will charge faster than the ipad 2 if you use the 12W charger.