That depends on how concentrated the activated charcoal in your charcoal toothpaste is, as well as how healthy your teeth are, to begin with. Individuals with a thin enamel layer on their teeth should probably avoid using charcoal toothpaste altogether, or use a less concentrated charcoal toothpaste brand, such as those made by Crest and Colgate.  Even if you have a healthy layer of enamel, charcoal toothpaste should be used for teeth whitening every other day at most. Less concentrated charcoal toothpaste brands can be used with greater frequency.