Given different voltages, it gets a little trickier...
A 36v 20ah running a 250w motor = a little less than 3 hours. (see above calc)
My 500w hub motor kit came with a 36v10ah battery and was advertised to go 20 miles on a charge with a top speed of 20mph. But 360 watt-hours (batt) / 500 watts (hub motor) = 0.72 hours and 0.72 hours x 20mph = 14.4 miles! (My maximum range on a charge) Did I calculate right or did I get taken for a ride?