When do faint lines get darker?

Abbie
Hi all! 
​I went to the doctors yesterday as I was constantly  feeling nauseous, he gave me some tablets but highly recommended I take a hpt before taking any tablets. Well I bought a clear blue (not expecting to see anything as 5 days until period!) and immediately a faint line appeared! 2 more tests later and the faint line has showed on all 3!! :D (still in absolute disbelief!) I'm still 4 days until my period is due but wondering when I should expect my line to get darker on a clear blue? 
​Thank you!