Faint OPK lines

Ka
This is my first month using OPKs, and for a while I wasn't getting any sort of test line at all, just the control line!
Then about 4 days ago I started seeing the faintest of faint test lines, but I'm in my fertile window, with ovulation predicted to be just a couple days away. 
How quickly do OPKs go from being suuuuuper faint to positive? Could it still be a week or more? Or does it mean it's getting closer?