So I've just started using OPKs and I was wondering if it is normal for the line to not show for 5 minutes?

I keep taking the tests and there is a vvvvvv faint line until about 5 minutes when it darkens.

The pack says do not read after 10 minutes.

Also I have been keeping them for longer than it states. My line was very faint last week, barely noticeable and now it is starting to dry much darker than the others. Is it likely that I'm coming up to ovulation/my fertile period?