[BrianG]

Generally, if you stay at around the same total rpm with different motor/battery, runtime will be about the same. The same amount of work is being done.

If you increase voltage and motor windings and use the same capacity battery, runtime will increase. Just keep in mind added voltage usually makes for heavier batteries, so that will affect runtime a little. But the reduced current from a higher voltage setup makes everything run cooler and more efficient, which could counter the added battery weight.