OPKs and length of surge

This is my first month using OPKs. I've had positives for the last 3 days. Does anyone know if it's normal to last that long? I thought I would only have 1-2 positives. I'm just wondering what this means for my ovulation. Do you think ovulation occurs in the middle of the surge or at the end? It says ovulation takes place around 24 hours after the initial surge, but wouldn't I have a negative after ovulation on the third day?