On a dle20 with a 14 oz tank I flew 15 minutes and still had half a tank, and thats with an APC 16x8 prop, mostly WOT.  It also depends on how light the wing loading is. If the plane can hover at 1/8th throttle without stalling, chances are you can fly even longer.  My 4*120 will stall out at that speed, a touch over 1/4 is where mine is at.

Funny thing is, between 1/2 and WOT there is not much difference in RPMs!