Gosh I really have no hard and fast rule on it besides the visual look over and asking my kids. And everyone sweats in varying degrees. Ick.
It also depends on what kid and who's jeans.
I mean, my 11 year old daughter versus my 7 year old typical boy... well, they each have different degrees of dirtiness and stinky-ness etc. and activities.
Then, versus the Husband and his jeans.
Typically, 2-3 times. But again, I have no rule on it.
Also by now and with the ages of my kids, I will ask THEM and tell them, if they think or know their jeans/shorts are dirty, then PUT it in the laundry basket. But I tell them wear it more than once. Unless they were having a relationship with the dirt that day.
My son doesn't care if something is dirty or not...so with him, I will show him "look, this is dirty. See...." so that he can, learn to gauge it too. Don't want him getting married and still not able to do it. LOL
My Husband, can literally keep wearing a pair of his favorite jeans until eternity. Ugh. And I tell him he needs to put it in the laundry basket or it doesn't get washed.
If my daughter feels her jeans needs to be washed after 1 wearing, fine. It gets washed, she puts it in the basket. I trust her judgment. She knows.
Same for me and my jeans.
If I went horseback riding with a pair of jeans, versus wearing it just to go to the mall, well, I wash it accordingly as need be.
So anyway, I have no hard and fast rule about it.
I'm not a jeans police. Hopefully all family members can discern jeans dirtiness too. Not only Mommy! LOL