Ovulation test questions HELP!!

For those who have used, or are using the Easy at Home opk strips, how soon before ovulation do you see the stops getting darker?

I had a fairly dark, but not positive, opk on cycle day 8 (2 days after the end of AF) and the tests have just been getting lighter, to the point that the test line is almost gone completely. I am at cycle day 13 today. Is this normal? Will they start to darken again? According to my apps, ovulation is projected for Monday the 25th. Am I just too early?