Keep, yes, sperm definitely hang around. My last BFP was from DTD 6 days before ovulation, DH then struggled with finishing in the days leading up to ovulation, so it was just that one time, as he didn't manage to DTD again until after ovulation. I've heard people say they've got pregnant 7 days after too, but never heard anyone manage it any longer apart than that.
From what I can gather, from a rmc thread on here, summarising the leading experts in this field, as long as there is sperm waiting around, chances are a sperm will fertilise the egg, but then it's a question of whether the egg develops properly or not, which we can't control, and then whether it implants properly or not, which doctors can help control. And if the lining of the uterus isn't working properly, it can either be over selective (and so may reject perfectly good eggs from implanting leading to chemical pregnancies/early miscarriages) or under selective and let any old crap implant, which will then not develop properly and lead to a mc. So whilst we all focus on having sex loads and as close to ovulation, all that seems to matter is having some sperm lying in wait, and we can't control the development, but we can take progesterone to help prepare the lining to allow implantation, or to reject the duff eggs. I'm going with this theory so am not pressuring DH to DTD all the time, just focusing on every 4-5 days if he can! Then take progesterone and clexane in due course!