How regularly is it safe to dye your hair?

Hair dyes have not been shown to be harmful to people's health, although there are some who worry that the ingredients might not be totally safe.

As for the hair, itself, the hair isn't alive. It has no blood supply and the normal repair mechanisms used by the body don't apply to the hair. You can dye it as often as you like. However, as you continue to dye it, it will accumulate damage and eventually break when the damage overcomes the backbone strength of the hair. For that reason, when you retouch, you should only retouch the new growth.