20ma.h is your battery capacity. Covert that over to milliamps per minute and divide it by the flight time.
(20ma.h)*(60min/h) = 1200 ma.min
Since lipos don't get fully discharged, and say 80% of total capacity was used, this decreases the usable size of the battery to:
1200 ma.min*0.8 = 960 a.min
The flight time was 12 minutes so:
(960 a.min)/12min = 80 ma
And that's the average current consumption of your motor. An equation to find the current consumption of any motor based on battery capacity and flight time would be:
Note: 60*0.8 = 48
(C*48)/T = A
Where:
C = Battery capacity in ma.h
T = Flight time in minutes
A = Motor current in milliamps
Google search works as a good calculator. Stick in the Equation defining A and substitute the variables in the search menu and it will give you the result.
Hope that helped!