Holding a charge and drain due to load are two different things.

I use a 1200 6 volt pack with 5 servos and typicaly will get 5 ten minute flights before I consider a re-charge. I have a voltwatch on board that allows me to keep an eye on the battery condition before and after each flight. When it begins to get out of the green and into the yellow while I move the sticks, it's time to recharge. That usually happens after my fifth flight.

Your setup is more ideal because you won't have an engine vibrating which causes the servos to gitter and consume more current. And, you only have two servos. You will definitely be able to get at least an hour, and as suggested, as much as two. And, again, as mentioned, your neck will hurt long before you have to land because of your battery.
CGr.