I just graduated college, and my usual rule was I didn't call anyone after 9:00 unless they were really good friends, family, or I needed something from them regarding homework due the next day, in which case the deadline extended to 10.
I had one friend call me at midnight once, and I told him that the next time he called me at midnight, he better be seriously bleeding or know someone who is. The only people I've ever given the "you can call me any time...no, seriously" privilege to are my family, my best friend, and my ex-boyfriend. Well, he wasn't my ex-boyfriend at the time...