How much range you get depends on two factors: 1 How many watt-hours you have; and 2 how fast you use them.
If you used only full throttle and had 15 amp controllers in each , they would last equally long in time, i.e. 13/15 = 0.87 hrs, but you'd be going faster with 48v because you have more power, so you'd cover more distance.
If you had two identical bikes and riders, one with a 36v battery and one 48v, both 13AH, and you rode side by side, the 48v battery would go 30% further.
If you use the increase in torque, it will use up watt-hours at a faster rate.
In practice, you can keep the controller set at 15 amps. When you're riding, you can adjust the power with the throttle and/or PAS setting, so go as fast or slow as you want, but at least you'll have the extra 30% power when you need it.