With that tank you should get at least 14-20 minutes of flight time as the engine only consumes about .6 to .7 oz/min at wide open throttle.

That's assuming a properly tuned engine too. An untuned engine can toss away unburned fuel at a high rate.

As stated check the clunk and do a ground engine duration timing test.

Once the engine quits check the tank to see how much fuel remains.

If there is still a good visible amount of fuel you know the answer....