Longer. Here's how it works. An LJ surge means your bod should release an egg withing 24-72 hours. The fastest you could possibly expect a BFP is still many days away. It takes up to 24 hours for fertilization, another three days for implantation, and a further five days for there to be enough hCG to be detectable. It's not at all uncommon for women to still get BFNs clear up through when their periods are due, or later. It's often advised to wait until the first day of a missed period, and if it's - and no period still, then to wait another week. If there's still no period, take another test. It's tempting to start testing every day right away, but that stress is counterproductive.