do NOT put them in the washing machine, or even wash it with water, because that will remove the chemicals from the cleaning cloth.
The only way it is used up is when it has gone completely jet black, however, it generally takes about a few years of use for them to go completely black, unless you clean them as often as jewellery stores do (which is almost everyday or once a week).
I would definitely recommend you to replace them instead of attempting to wash it and removing the chemical and therefore damaging your pieces of jewellery.