How long does it take to trust someone new?

I don't trust easily. I don't go around acting all aloof and stuff, but I keep relationships with friends light until I feel I can trust them. I've had a really hard time with friends over the years. It started when i was very young. I just couldn't keep a friend longer than 9 months or so. My high school "friends" were ok with me hanging around them at school but hid the fact that they had parties several times a month that I wasn't even to know about much less be invited to. I had good friends in college but we live across the country from them. My adult friendships have not been any easier.

If someone is my friend I expect them to either dump me or move within two years.