How many days after your period is it safe to have sex with a little chance of getting pregnant?

Tricky Timing: Ovulation perhaps most reliably occurs 14 before the menses. Ovulation can vary by several days between cycles. The egg can be fertilized for about 18 hours after ovulation but the sperm can live inside you for 2-3 days or more. If you really do not want to be pregnant then use a contraceptive in addition to timing. Timing failure rate is about 25%/year.

Never 100% safe: If you're irregular, then you can never safely avoid pregnancy. It's possible, although unlikely, to have a period on one day and then to ovulate 40 days later. If you have a track record of having very consistently predictable cycles, then it is feasible to use timing as a form of birth control. It's not 100% effective, just as no method (other than complete abstinence) is 100% effective.