Peak plasma concentration of Prozac occurs 6-8 hours after taking the drug. It works best when taken every day for 2-4 weeks. Prozac has a long half-life of 2-4 days, meaning that it takes that long for the dose in your body to be reduced by half. As Prozac is broken down, the resulting substances, called metabolites, stay in your system even longer—at least 7-9 days before half of these substances are gone.