No experience with the Zenoah, but my US 35 ( I know, overweight and under powered but it was cheep.... they say the same thing about me ) would easily get 20-25 minutes.

I actually time all my engines on the ground at full throttle, then use a kitchen timer, and subtract a couple of minutes for safety, and I don't usually fly at full throttle for the whole flight.... another little margin of safety. I like to have at least 1/4 tank when I land...... big birds cost way too much, and I make way too little to lose one to running out of gas at an inopportune time.

My two cents.

Bob