I have heard its 7 days after ovulation/conception. The thing is HCG (the hormone home preg. tests test for) does not show up in your system until the embryo has implanted in your uterus which can be 7-14 days after ovulation/conception. It varies big time and so do the home pregnancy tests. Some can pick up lower HCG levels than other. That why the basic rule is to wait till you miss your period but I have and I know others have tested earlier than that.
I tested about 10dpo and got a negative, and then about 12dpo and got a +.
I also read somewhere that it can take a few days after implantation for enough hcg to show up for the test to read +.