my lines got darker pretty fast as they started at 8 dpo. but as my pregnancy progressed, id say at about 8 weeks along and more, my pregnancy tests were so overwhelmed with hcg that they showed faint and the longer i held my urine in, the fainter the test would show. this is normal but i did worry at first until i found out that high levels can send typical home tests flying out of their senses. oh, and i kept testing because i had alot of tests soon to expire and wanted to use them up!