Does anyone know what flight time I would get with a 3W150 and a 20oz tank?

Probably not much. I would get 15 minutes with a 40oz tank with my ZDZ 120 twin.

I have a DA150 with a 50 oz tank. Most of the time I run full throttle on a 42% Super Decathlon (75#). I set my timer for 12 minutes. Probably the engine would quit after 15 minutes.

I agree with Big_Bird. My BME 150 uses a 60oz tank which is good for a safe 10-12 minute flight.