This is a subjective question in that it depends a great deal on the individual. Some women have carried a baby to term without realizing they were pregnant (surprise surprise when the water breaks).
If you meant to ask when is the earliest that a pregnancy can be detected, newer home pregnancy tests detect very small amounts of hCG, some as low as 5 mIU/L. With these very sensitive tests, it is possible to test positive as early as seven days past ovulation. It is important to remember that implantation occurs anywhere from seven to ten days past ovulation and women produce varying amounts of hCG, eventually forming a pattern of doubling hCG amounts every two to three days. While it is possible to test positive as early as seven days past ovulation, there is also a good chance that implantation has not occurred yet or that you are not producing hCG at a high enough level to be detected by a home pregnancy test. For best results, it is still recommended to wait until a woman has missed her period or 14 days past ovulation. For those that just can't wait, the very earliest you should test is seven days past ovulation.1