Ovulation question

Hey ladies. I took an ovulation test last night and the lines were pretty dark. I took one again today to see if the lines were going to be close to the same in darkness, however they weren't. Still had both lines today, just wasn't as dark as yesterday. When I put in the test to my log it changed my fertile time frame and I have like 2 days left (today and tomorrow).

My question is, how long can you test positive for ovulation and does it usually only last for about 24-48 hours?

The top test was last night and the bottom was today. If that helps at all.