When did your lines darken?

I found out I was pregnant yesterday with a faint line on a First Response. I took another test this am expecting it to be much darker of a line and it wasn't. My period isn't expected until Sunday, so I know it's early. Pretty sure conception was the 31st of March. I'm just wondering when everyone's line became more prominent. I found out I was pregnant with my last child the day before my missed period and by the next day the line was much darker. Looking for reassurance!!