Many of the hpt's on the market today can detect the pregnancy hormone as early as 5 days BEFORE your missed period. Therefore if you wait until the day you period is expected and it does not show you can take a test and it should be accurate if you have regular periods. If you do not have regular periods you can either wait 2 weeks from when you ovulated if you know or you can wait 2 weeks from the time you had intercourse.
Since I had irregular periods, I waited two weeks but if yours are regular I would say after 1 week.
As soon as your period doesn't show as expected you can take one.