You can take ibuprofin four times per day - every four to six hours as needed. The half-life of ibuprofen is approximately 2 hours. This means that after 2 hours, only half of the active dose is still present in your system. After 6 hours (3 half-lives) there's 1/8th left of the original dose in your system, which is not enough to perceive any effects. After one day (12 half-lives) there's only 1/4096th left, or practically none.