In a fertile environment sperm can survive for up to 5 days! If you are not close to ovulating, thus your body is not in a fertile phase, the sperm won't last more than a few hours.
Sex about every other day should do the trick. But it would be much easier if you knew when your window of ovulation is, then just time sex with that :)
Here is a link where you can see when it is likely you will ovulate... good luck :)
American Pregnancy Association : Getting Pregnant: Ovulation Calendar