Is your BFN after AF or after you think AF is due? If it's the former, then you're likely not pregnant, but if it's the latter (eg day 33 of a 28-day cycle) then there's a chance you could be pregnant. Some people have taken weeks and weeks - 7 weeks in some cases. Pop along to your GP and ask for a blood test if you keep getting BFNs and no AF.

HTH.