a positive OPK means that your LH surge has started. LH is the hormone that stimulates the ovaries into releasing an egg. The surge usually lasts 24-36 hours. Most woman would release an egg in this time. And the egg can last from just 6hrs to 24hrs before it becomes useless.
So the only real way of knowing is to do something else along with using OPKS like taking your temp. Your temp drops on the day you ovulate quite noticably, then rises the day after. You can also check your CM but a lot of girls find this confusing unless you do it for quite a few cycles and really get to know your body.
either that or just follow the SMEP plan to make sure you dont miss the egg!!