depends on what you want,

my avistar had the factory 10 1/2 oz tank with an o.s. 40LA. while i probably shouldn't have, i flew at full throttle most of the time (a habit i've been trying to break ever since. ) i could fly 15+ minutes on a tank of fuel. after i traded it to another flyer he made the remark that "this plane makes its own fuel, i'm never gonna run out."

one other thing, what type trainer do you have?