A good rule is to wait until your period is a week late. Those tests aren't cheap and it is a waste of money to test and then have your period start the next day.
Yes Moga is right. Wait till you are a week late. If you do a home test and it comes up positive make an appointment and see you doctor for a blood test, if that comes up positive your 99.99% sure you are positive.
Wait to see if you have a period.