I would count how many pumps of fuel you put in, fly ten minutes and land. Then count how many pumps you take out. This will give you an idea. Then keep going about one minute more on flight each time you fly and recounting the pumps you take out to strecth your flight time out.

On my nexstar, and with my style of flying, I fly 14 minutes and then land. Mine was also the RTF with the O.S.46Fxi.