I personally always change my pants everyday so I wash my pair of pants everyday when I change clothes.
I have gone 2 days before though without changing my pants or washing the pants when I wasn't feeling too good.
So I would say as long as you're washing your pants after 2 to 3 days of wearing them then that should be good enough but it's best to change into clean pants and clothing everyday and wash the pants and clothes again before wearing them just for good hygiene.
You also look presentable when wearing clean clothing especially when looking for a job or when going to work.