When should you test??????

Just a little back story about my situation as not everyone's is the same. I have had one pregnancy and it went full term. I was diagnosed with pics when I was 19. We have been ttc since September 2012. My period use to be very irregular until my first round of clomid about a year ago. Since then my period had been on time give or take a day. But I am now a week late. After so many bfn I don't want to test too soon and get a negative when it could be a positive. So any suggestions on what is the best time to test?