After wearing jeans for even one day I change into a new pair of jeans and then wash the old jeans.
However I have worn the same pair of jeans for a few days before over the weekend when I have been busy and tired of doing anything else so I just changed into a new pair of jeans by Monday morning and then washed the dirty pair of jeans.
As long as you're not getting too dirty then it's okay to go a few days without washing a pair of jeans and wear them for a few days.
However after a few days of wearing the same jeans you really should change into a new pair of jeans just to keep clean.
Your not gonna be harmed though if you do decide to wear the same pair of jeans for a few days or so however you'll feel much better after showering and then changing into a fresh clean new pair of jeans after a few days.