How long should I wait to take a pregnancy test?

I stopped the BCP in May of this year but my cycles were irregular. My OB put me on provera for 7 days and AF came on 9/17/09 and lasted for 5 days. Well here it is Oct 24th and no AF. I did the OPK and according to the test I was very fertile on Oct 3rd. The hubby and I BD 2 days prior, on the day of (oct 3rd), and 2 days after. I am 38 days into my cycle and nothing. I tested on oct 15th which would be my 28th day in but I received a BFN. I was told I may have tested early so when should I test?