After each time I've worn them.
TBH I only wear jeans maybe once or twice a year so it's sort of irrelevant to me, my shorts though I tend to wear 2 or 3 times before washing (depends on what I've done in them).
With shorts which I effectively wear whenever it's not cold I don't have to worry about heat or weft so just let skid size be my indicator, commando obvs.