How long you should wait to take plan b?

As soon as possible: If you do not want to get pregnant, take it as soon as possible after unprotected sex but definitely within 3days or is useless.

Don't wait: 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.