How Long Does It Take a Crystal Meth User to Start Picking at Their Face?

I have a friend who is supposedly been clean for the last 8 months was a junkie but claims she has had no drugs in her system for that long. She went away for a few days then came home very aggressive looks like she has picked sores all over her face and bad mood swings. I think she has been using for a while but cant prove it so i would really like to know when the crawling under the skin starts to make you pick at your face.