I wait until they can stand up on their own then I know its time to change them.....
JUST KIDDING.....
Not a germ o phobe either so it's usually twice a week, more often in the humid months. If you've done a good job in the shower, what's going to get on your towel that's so horrible? If your afraid of some dead skin cells, you would never sleep in your bed, sit on your sofa, wear your bathrobe more than once, go any place that's public for that matter. Its just something you should not let your mind explore.