I don't use a washcloth. I reuse the towel I dry with for at least a week, until the next time I do laundry. I think it's extremely wasteful (not only of water, but of gas and/or electricity) to change towels more often than that, plus I see no reason to do so. Towels will also last longer and need to be replaced less often if you don't wash them too often.
I travel on business 2 or 3 times a year, and most hotels I stay at participate in a towel & linen reuse program, such as the ones sponsored by the American Public Health Association and Project Planet. There's a sign or notice in the room letting guests know how much water and energy is wasted washing towels, plus adding detergent to the environment. Then it instructs you to hang your towel if you reuse it, or to leave your towel on the floor or in the tub if you want it replaced.
Interesting, I read a report on the success of the towel reuse program, and it turns out that guests who used an American Express card reused their towels significantly less often than guest who used a MasterCard or Visa. I guess being environmentally irresponsible is one of the privileges that come with AmEx membership.