Which towels are we talking about, and how frequently do they get used?

Bath towels, hand towels and dish drying towels will all get dirty at different rates, and get/stay wet at different rates.

Towels should smell clean (clean, not perfumy) and be dry and not feel like they’ve got something on them. The more time a towel stays wet, the more often you wash it. If it gets noticeably dirty, you wash it. This could be anywhere from once a day to never, if it’s just decorative and you never use it.