Ovulation tests

When you get a positve on an ovulation predicter kit when do you actually ovulate? 12-36 hrs later or the positve means your ovulating and it only last from 12-36hrs after you get the initial positve? We have been trying for over a year and I am starting to wonder if I am just constantly missing my days. I guess my question is also what days do you have sex in order to conceive?