Information on Plan B;
The sooner you take it the better. The risk of pregnancy for the one episode of unprotected intercourse is 0.5 percent if the pill is taken within 12 hours; the risk increases to 4 percent if Plan B is started 72 hours after exposure. The FDA approved Plan B to be taken up to 72 hours after exposure, but some studies have found it can reduce pregnancy rates even if taken 120 hours after having sex. In general, when compared to a placebo, Plan B prevents 80 percent of pregnancies if taken in the “up to 72 hour” time frame.
Found here: http://www.google.ca/url?sa=t&rct=j&...bODOvQ&cad=rja
In other words. You should take it within 72 hours of unprotected sex, and the sooner the lower the odds of pregnancy. But some studies (and those studies are subjective) show that any time before 120 hours can reduce the risk, just not as much as it would if you took it immediately, or at least within the 72 hours time frame.
Of course I'm sure you know that even with the Plan B, even if taken right after sex, pregnancy is still possible. Like birth control, plan B only reduces the odds of pregnancy. It does not claim to completely eliminate the risks of pregnancy.