Women are fertile to concieve 10-14 from the first day of their last period (lmp). Ask her when the first day of her lmp began and add 10-14 days. Women should ovulate on day 14 but many situations cause early or late ovulation. The time frame starts at 10 because sperm can live 72 hours in a woman before dying (though a majority die within the first 24 and the strongest fighters last til 72).

Also, don't rule out second ovulations during one month. But I have a feeling if you didn't cum in her, then it should be ok but not positive.

Good luck.