[GasperG]

Can you point me to the regulations where it says that stint length can not be exceeded and that this is measured in the number of laps?

For the general people to know why the penalty it's easier to name this "stint length" penalty, but I'm 100% certain that this is based on fuel allowance per stint and not km driven in "race mode".

Fuel usage penalty are actually two types, fuel per stint and energy (fuel) per lap, maybe just maybe "fuel usage penalty" is measured per lap fuel energy allowance and "stint length penalty" is stint fuel allowance. As @carbsmith noted per regulation there is min amount of fuel that must always be present in the car, so even if the car uses all the fuel that is alowed per stint it still has a litre or two of fuel so it doesn't get stranded.