I usually have my students fly no more than about 11 minutes. This goes to what I said in the other thread you have running, that's about the max attention span when learning. One way to find out how long you can fly is to simply run the engine on the ground. Fill it up with fuel, start it, open it up full throttle, and time how long it will run on the tank of fuel. You can then base your flying time on that. I have a simple countdown timer (I found mine in the baking section at Wal-mart) to time you flight, simply set it for the time you want to fly and then an alarm will sound when it's time to land. I prefer this to a stopwatch because you don't have to look at the timer, just listen for the alarm.

Hope this helps

Ken