The aproximate amount of time since last time you had sex/intimacy with someone; when you are the most in danger of doing something stupid just because it's been a while since last time you got any.
Often involves going back to previous partners.
If you get over the 2 & 1/2 week rule phase, your next sexual encounter is much less likely to be looked upon as unfortunate.
-What? Jake went home with Mary again last night?!
-Yeah, he fell victim to the 2 & 1/2 week rule.