So you mean with unlimited fuel and ignition battery capacity? You could fly until the plug fouled if you tune the engine properly. If it's too lean and quits after 20 minutes it's tuned wrong. Tune it right, keep it cool, and it can run all day.

By the way, sometimes I fuel my DA 50 up right after I land and I go up again. I do that for 3 flights in a row. no problem at all. That's 30 to 45 minutes right there.