How are you making your determination that the flight should last 20 minutes? The best way to estimate flight duration is to divide the capacity of the battery by the average current drawn by the motor. Full-throttle current could be used as a "worst case" scenario.

Do you have an idea of how many Amps this setup should be drawing? If not, how many cells are in the pack?

Overloading the pack will cause it to lose power quicker than expected. What is the C rating on this pack? Ebay packs are often overrated, or of poor quality unless you buy name-brand packs.

Does the ESC have a programmable cutoff? What is it set at?