@Spandauer it goes for about 2 weeks, but you can then refresh it. The original took 2 hours, a refresh will take about 45 minutes. The hair regrowing changes how it looks, but it still works.
DD is planning to grow her hair after Halloween (she wants skulls for that) so I'm enjoying my last hurrah. We've been doing this for about 18 months now and I've got good at it. Painting on hair is a whole new skill set.