Pat

I fly 12 minutes, land with 1/4 of a tank...but I'm very light and short on full throttle, take off I slooowwly open the throttle and its always airbourne before I reach full power and I stop advancing then unless I'm going to pull over the top and 1/2 cuban.
Scale flying 15 minutes is safe, German/American screaming around the sky at Mach 1.2 :-) is 10 minutes safe with a P120SX.
No need for more fuel, just the stock tank and UAT does just fine

regards

Dave