How long do riding lights stay on when batery rubs out?

[Ed57]

Yesterday I did a 54 mile ride on my Gazelle T10 and ran out of battery 3 miles from home (there are some big hills on the Shropshire Powys border)
I was supprised to find that the lights were still worked. Is this common on all Pedelecs?

 

[soundwave]

bosch leave a bit in the tank batt wise if you have a light fitted to the motor