How long black magic lasts?

I believe that black magic lasts until the job is done, or until it is repelled. black magic from voodoo, otherwise known as woodoo, is deadly, and most of the time lasts forever, like turning someone into a zombie. witchcraft black magic is bad too, but it always backfires in someway, such as if you want someone's apartment, but the guy is old, you use black magic to make him crazy, and he moves out. but then when you move in, the ghost of his wife shall haunt you forever.

It will probably last until people stop believing in it. The same as other superstitions. give me valid proof that it is a superstition. Friday the 13th is a superstition (not the day, but the many modern beliefs.)