How quickly should pregnancy tests progress?

My cycles have always a little irregular between 31-35 days. Anyway on Monday CD 25, I took a frer as I had a few symptoms and got a vvvfl. This would be approximately 6-7 days before I'd expect my period although I can't be certain due to their irregularity. Took another frer today CD 28, and there's still a faint line. It may be slightly darker than it was on Monday but not much, and negative on CB digital. My question is, how much would you expect the line to progress in very early pregnancy. At this stage, would only a small change still mean that it's a possible viable pregnancy? Thanks all.