Hi Scale,
I've got two extras. One completed with a Moki 2.1 and the other waiting on a gas engine (Hopefully the BME 50 if I can find one at a reasonable price). The Moki usually flies 12-13 minutes, semi hard but not full throttle all the time and I use about 2/3 -3/4 of a tank. Depending on the flight time you want, I find this size tank acceptable.