It depends really. Women who are not pregnant have between 1 and 5mIU/ml of hCG in their urine, and as soon as implantation occurs the placenta starts to produce more hCG, which generally doubles every 2 to 3 days. Implantation is usually between 6 and 10 days after ovulation.
The earliest you could see a BFP would be 8dpo - if implantation occurred at 6dpo, you already had 5mIU/ml of hCG, which doubled every 2 days and you tested with a sensitive pregnancy test (10mIU).
On the other hand, if implantation didn't happen until 10dpo and you had 1mIU of hCG, which doubled every 3 days, you wouldn't see BFP until 20dpo, nearly a week after a missed period, even with a 10miu test.
Obviously, there will be people who saw a BFP earlier than that and people who don't see one until later. Not all bodies work exactly the same.