I used to get 4 days of positive opks. I also charted as well. After doing 3 months of both I got the hang of when I actually ovulated. It was always the day after the darkest positive (so usually after the second positive opk). When I get onto my computer I'll link my charts so you can see what I'm talking about. I read all these horror stories online about people who get lots of positive opks in a row (like "building up to ovulate, but not actually releasing an egg" ). Totally freaked me out, (so stay away from google!) but got pregnant first month trying, so even with four days of positives you can accurately predict ovulation... Hope that helps