How long does gun residue last on hands?

Until someone breaks it, otherwise years. most of the time the problems with Nerf guns are easy to fix.

It depends on how often you masturbate.

They last forever. Or at least until you die and your body rots.

a cheap one with a little care can go for abotu two years without needing new parts. High end guns can last as long as you dont physically break them.

That will depend on the type of gun (revolver versus rifle, etc) how many times it was fired, and what the hands were exposed to after the firing. It is typically measured in days.