Ovulation test question

THE Brittany 🦄 • Railroad Wife 🚂 Mommy to Ava Grace (11/28/15) Colton David (3/15/19) #3 (our angel 👼🏻) and #4 Layla Rose (10/28/21)

Are OPKs like pregnancy tests where you have to read them within a certain time frame? The tests I have take some time to get dark but I don’t know if that means they are invalid? 🤷🏻‍♀️

Also I know this is negative. I’m just posting as an example.