Some pregnancy tests advertise that they are 99% accurate even after a week or so... so I'd test ASAP..
It's up to five days before your period. And the 99% accurate is if you get a positive result. You can actually test whenever and the test will be 99% accurate as long as the results come up positive. Even on the early results test they recommend that for best results you wait at least until the day that you're suppose to get your period.
If you read through the pamphlet that comes with the HPT it'll give you a percent of women who are pregnant who test positive at five days before. That is something like 50-60% depending on the test.
I feel like a know it all... I've just retained a lot of what I've read and I think I've read just about every little pamphlet that goes through the bathroom.