Several factors determine flight time on a tank of fuel. One obvious one is the size of the tank. Another one that is less obvious to some flyers is throttle management. If you fly with your hair on fire, throttle wide open, then expect short tank duration. If you are more cautious and reasonably easy on the throttle, then it will last longer.

I have an OS .75 AX that I've run for 10 minute timed flights. So far, I've used about half of a tank per flight. And that's on a Skylark 70, which is a good sized plane for that engine.

So, it's difficult to just say 'you will have x minutes per tank' when it's difficult to determine your flying style. There is nothing wrong with 'hair on fire' flights, it's just you will not have good fuel economy flying that way. If you like going fast, then that's the price of it. If you fly more 'reasonably', then your tanks will last longer.

CGr.