I would say for everyone is different. I've got my BFP 6 days before my AF was due (it was in March though), just had quite a few symptoms and a sense that I should test (even though I thought it's way too early, but just couldn'r wait. My impatience won). So I've bought ASDA brand twin pack and tested at lunch time, I saw a very faint line... After an hour I've done another test and it was the same, so then went and bought CB digital and did it next morning and it showed 1-2 weeks pregnant...
But I guess I was one of the lucky ones to find out soooo early as many ladies here didn't know till quite a time after they've missed their AF. So I would say it's really 50/50.