How about never?
I have prescription -1.08 or something. That amounts to things being difficult for me to see clearly at about half the distance of someone with perfect eyesight.
And since that's distance vision, that's rarely an issue unless I have to look at something several meters away that has small details that matter.
I actually only discovered I had a problem at university when I couldn't read the boards and projector screens in lecture halls and classes.
(Which tend to be further away than those in classrooms earlier in education.)
So... The idea of wearing glasses in the bathroom would be pretty unlikely for me... I don't wear the glasses unless I really need them for something.
In fact, if I don't need them, they're a serious nuisance.