How long does the Japanese hair straightning perm last?

don’t do that to yourself.

Is it badd for my hair though

It chemically alters the structure of your hair, permanently. And it’s probably caustic, so I think it’s bad for you.

It lasts until your hair grows out, but the way it works is that it breaks down and then restructures the bonds of your hair chemically, so there’s a danger (depending on what sort of hair you’ve got) that the treated hair can break off when the new growth comes in.

Plus, the process is hideously expensive, it must be touched up every 6 weeks, depending on how quickly your hair grows, and the practitioners can be people who just had a one-day course and don’t really know what they’re doing…