When is it time to take a shower

Every day, at least once. Twice a day, if you sweat a lot in summer.

When you can smell yourself it’s already too late

After work, (at least in my field) then I do it before I go to bed on the weekends

anytime

Imo, do it often enough to avoid bad odor, fungal infections, too dry/oily skin, or any other negative result of not having taken a shower. But don't shower when it's not necessary.

Preferably before thou stinketh.