Implantation of the fertilized egg usually takes place at around 7-10 days post ovulation. Though it can occur as early as 4 days and as late as 12 days. When you chart your cycle you will find out how long your luteal phase is - this will determine when menstruation should begin and thus should tell you when to take a test. However, if you are real attuned to your body, and can identify possible pregnancy symptoms before the menstruation is missed, you could see a positive pregnancy test sooner.
This is true of myself. When I found out that I was pregnant with my forth child my menstrual cycle wasn't due yet. However, I had some classic pregnancy symptoms such as food cravings starting at 5 days post ovulation. At 11 days post ovulation I tested and it was positive. It was positive again the next day. I probably would have gotten a positive sooner.
There are many different brands of hpt's that detect as little as 20 mIU/hCG (human chorionic gonadatrophin, the hormone produced when implantation occurs.) Below is a chart that shows averages of hCG starting with week 3 to week 40.
Week from the Last Menstral Period (LMP) Amount of HCG in mIU/ml
A woman can test as early as 8 days post ovulation and receive a positive pregnancy test. On the other hand, it can take up to 17 days to see a positive hpt. And some women, though few, will never see a positive hpt - even though they are indeed pregnant. I have a good friend who never had a positive hpt with all three of her children!
The next decision is to decide which brand is the best pregnancy test to buy. I personally have tried just about all the brands that I could get a hold of. I have heard rumors that some are not as reliable as others and some may result in a false positive pregnancy test.