It depends on how much load you have on the battery and the actual C rating of the battery. If you want your batteries to last a long time, you should only run them to 3.7 or 3.8 volts per cell. You probably won't find a cutoff in that range so you will have to time your runs or take an educated guess. It's better to stop and charge them at 3.9 then 3.5v.