It can take up to a year of trying before we consider fertility problems.
Try to time having sex to when she is most fertile. Her peak would be between days 11 and 16 of her cycle, day 1 is when her period starts. Sperm can survive about 5 days so you'd want to start around day 6.
48 hour after she ovulates or by about day 18, the egg isn't viable. Her fertility is decreasing and she is unlikely to get pregnant.
So, from about 10 days BEFORE her period until 6 days AFTER her period starts, take a break.
Wear loose boxers too.