Oh my goodness, some of you lots are going to love me. We're a family of 2 adults and 2 children, we merrily share towels and I'm not sure how often we wash them. Maybe once a week, give or take. We don't have a towel-washing schedule. 
We all have a shower or bath every day and have several towels on the go so there are always dry ones available. We don't have a tumble drier so dry indoors / on radiators / outside on the line.
Any time there's space in a wash we grab a towel to fill it. If there's one that smells damp / musty then that's the one that goes in.
It's heartbreaking that there are so many people out there who clearly don't give a single shit about the environment. I presume most of these daily-washed towels are also tumble-dried for hours?!
Seriously, any bacteria that might grow in a few days on a slightly damp towel that's been used to dry a clean body are not going to do you any harm. In fact, they are probably essential to the working of your immune system.