It depends on the rides that you do. I have to think that the engineers who design the light will also make sure the battery power supply is adequate for a wide range of circumstances.

So the question about what's the longest before recharge is about what's your ride. Recharging some of these lights is as simple as plugging in the USB cable to a 2.1 amp charger like Belkin. I say "so what" about recharging frequently. Just do it the night before.