I tested this week on Sat, Sun and Mon. On Sat I didn't think there was any line at all, then on Sun it was about as half as dark as the control line, and then yesterday it was much darker than the control line.

It probably depends on what brand you use too I think. I have the Lullaby OPKs. Peeing in the cup seems to be a lot easier than some I saw in shops where you pee on a stick and so actually have to aim! LOL