If i had unprotected sex and my boyfriend claims he didn't cum or precum. im ovulating. how long do i have before plan b is not effective?

Plan B: Plan B needs to be taken within 72 hrs of unprotected sex. If you take it within 72 hours after you've had unprotected sex, Plan B can reduce the risk of pregnancy by up to 89%. If you take Plan B within 24 hours of unprotected sex, it is about 95% effective. But you should know that Plan B is not as effective as regular contraception.

Efficacy: Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.