"If you have a test line that is fairly dark one day then very light the next, you may have missed the actual surge."
I got that from an OPK manfacturing site. I think you have about 24-48 from the beginning of the surge until ovulation. And then the little eggie is alive and viable for about 24 hours. So I'd probably say, go BD now lol!

EDIT: I second what the others have said too. Lots of factors can affect a positive OPK like time of day, water intake, and sometimes what is actually your surge doesn't get picked up on the OPK, for whatever reason. :-)