I think I have lice, but how long does it take from "getting" it to itching and everything? Please help!!?

I was camping with some girls, and I think one of them had lice. ANY irritation on my head makes me itch it, but I just think I'm getting paranoid about it now. I doubt I have anything, but I'm not "really" itching a lot or anything. Say if I had just one louse on my head, would I be itching?