I got pregnant on the pill, and I was religous about taking it.
It's hard to say with a hpt- usually, the longest it would take to show a positive result would be three weeks after conception- depnding on how sensitive the test is. Usually they say if you get a negative result when you are a week late for your period, you are almost certainly not pregnant, and should see a doctor to find out where your period has gottten to.
So, if you wait for about 3 and a half weeks from the time you had sex- it should be pretty accurate.
If you wanted to be more confident in the result, or your not sure about exact date- go to a doctor, and ask for a blood test. You could probably do this now, and not wait any longer to find out. He'd also tell you what to do if it comes back negative, and you still have no period-or if it comes back positive, discuss your options.
Good luck. :)