There are many theories about this, and no one clear answer. Sperm can survive in fertile mucus for up to five days. Therefore theoretically, conception can happen up to five days after intercourse. In practice, however, sperm usually last two to three days, not five, and even less time when there's no mucus. That's why the optimal time of intercourse greatly depends on the quality of your mucus (How to increase the quantity of fertile mucus?) and the quality/quantity of your partner’s sperm (How can my partner increase his fertility). In general, daily intercourse on fertile days and on the day of ovulation maximizes your chances of conception.