Research indicates that PEP can prevent infection with HIV, but it is not 100% effective. 1, PEP is more likely to be effective when the treatment starts shortly after the risk occurred, and most clinics will not put someone on a course of PEP any later than 72 hours after HIV exposure (risk). The sooner after the risk you begin PEP, the better the chance that it will work.

Rerence:
Roland ME et al. Seroconversion following non-occupational post-exposure prophylaxis. Eleventh Conference on Retroviruses and Opportunistic Infections, San Francisco, poster 888, 2004.