Generally, it's around 14 days from the first day of your period if you have regular cycles, but I was tracking my ovulation before becoming pregnant, and even though my cycles were usually 27-28 days long, I had cycles where I ovulated as early as day 11 and as late as day 18. The cycle I conceived, I ovulated on day 16 and had intercourse on day 15 (I'm pretty sure this is the day that did it as the only other time I had sex that fertile phase was 6 days before ovulation). But sperm can live up to 5 days in the right conditions, and some women have been known to ovulate very early some cycles, like around day 8.
If you want to avoid pregnancy, always use protection like the pill or condoms.
If you want to conceive, a lot of women start having sex at least every other day from when their periods end to around cycle day 20 to optimize their chances of conception.