Thank you. From the cited article:
"In Ipergay, gay men and other men and transgender women who have sex with men, and were at high risk of HIV infection, were asked to take two Truvada pills (or a placebo) from one day to two hours before they anticipated having sex. If they actually did have sex, then they were to take another pill 24 hours after having sex and a fourth pill 48 hours after it. The period of taking PrEP would thus cover two to three days. If they continued having sex, they were told to continue taking PrEP until 48 hours after their last experience.
My problem is I very rarely "anticipate" having sex. It just happens in the moment.