You should change your towel at least every 3 days although it's best to have a fresh towel every time you shower or bath to help prevent the spread of germs.
Although when you bathe or shower your body is or should be clean so it should not be much of an issue.
Still I always try to have a fresh towel for when I shower or bathe and I always make sure my kids have a fresh towel as well to dry off with.
But it's okay to go 3 days with the same towel and then wash it and replace it with a fresh towel.
The best way to prevent germs from growing on your bath towel is to let it dry completely between each use, and wash it frequently.
The Cleaning Institute recommends washing bath towels after three uses.
If you shower every day, that means laundry almost twice a week.
As a general rule, launder your bath towel (or swap in a clean one) at least once a week and your washcloth a couple times a week.
Wash towels more frequently if you're sick to avoid reinfection.