For many couples the most important information for trying to get pregnant is to answer the question: "When will ovulation happen". Fortunately, there are several methods which predict when ovulation will happen.
Ovulation Predictor Kits
Ovulation predictor kits test for the LH hormone which builds up towards ovulation. So as you start testing several days before ovulation you're likely to have a few with no line, then some with faint lines, and finally (hopefully) some with REALLY dark lines. Those are your peak readings (+OPKs) and they indicate that ovulation is imminent in 12-36 hours.
It doesn't matter when it turns negative again, and it is normally expected to stary positive for several days or only one day. Once it becomes positive you can stop testing.
Ovulation does not necessarily happen after it turns negative. It could happen, but that's not the rule.
What matter solely is when it OPK FIRST turns positive not when it eventually becomes negative. Ovulation happens within12-36 hours after the OPK FIRST becomes positive. What happens afterwards is not important. It could be positive for a day only or for 3-4 days. The length of it being positive doesn't predict when ovulation happens