Before we can answer this question we need to establish certain facts. Once the egg is released, it lives for only 24 hours. The sperm live 48-72 hours once released. If you miss this window of opportunity, you will not get pregnant that cycle. Trying to prospectively pinpoint the timing of ovulation (the release of the egg from the ovary) as close as possible is crucial, so that you can get sperm around at the time the egg is released.
Only 15% of women really get a period every 28 days, and normal ranges from 21 to 40 days. It's not all about day 14 of the cycle (unless you are one of those women who get a period every 28 days).
It is very individual, and you need to take your shortest cycle length and your longest cycle length and subtract 14 from each (for example if your shortest cycle length was 24 days and your longest was 34 days, you likely ovulate between days 10 and 20, with day one being the first day of bleeding). Once this is established, you want to make sure you are getting sperm around even before the egg ovulates.
I would recommend having sex 2 days before the window and every other day throughout the fertile time frame (for the above example, sex should occur days 8,10,12,14,16 and 18. If the sperm count is normal, you can have more frequent sex, but we recommend every other day because otherwise it can put too much pressure on a relationship and make sex a chore.
Alternatively, you can use an ovulation prediction test which will tell you when you are getting your LH surge, and the egg will be coming out 24 hours after that, so have sex the day of the positive LH surge, and for the next 2 days.