Runtime is dependent on a few different things;
Imagine your battery is like a bucket of water, and your motor is like a tap on the bucket. A bigger tap has the ability to drain the bucket faster than a smaller tap. If you turn on a big tap all the way you get alot of water coming out and it drains the bucket very quickly.
If you have a low turn motor and gear it too high it will drain your battery quickly, if you have a low turn motor and gear it lower, the battery will last longer.
Runtime also depends on how good your batts are, a 3000mah batt might only take 2500mah during charge and then only deliver 2000mah on discharge (there is always some energy lost in discharge, which is why they get warm)
New batts usually take more than their rating, eg some of my 3800mah batts take over 4300 mah during charge.
If you have a silver can motor and gear it right you will get heaps of runtime (at the expense of insane topspeeds)
Best way to measure runtime is with a stopwatch, makesure you get at least 6min.