Wash your dish towels every week. Kitchen towels collect food, and food stains may lead to bacteria and odor. If you cook often, you may want to wash your kitchen towels every week. If you cook less often, once a month can be enough. It also depends on how many people are using the kitchen. The more people, the more often you need to wash your kitchen towels. And if you have toddlers/young kids who get their hands dirty often, than washing them a few times a week may be needed.
Your decorative kitchen towels, however, only need to be washed once a month. Some people cook more often during the holidays and need to wash towels more often to have fresh ones out when they are needed most!