For me, it was actually just a few days... I think... Thing is, I'm not exactly regular. I'd vary from 24 days to 29 days. When I noticed I was at 31 days, I figured something was up. I got a test at the store while grocery shopping and was positive both that night and the morning after.

With my first baby, he was a surprise, I was late by about a week so I got a digital test and it was positive.
With my second baby I was three days late and got a + ( my cycles are 28 days on the dot so I knew something was up).
With my third baby I tested 5 days before I was due, it was a faint positive so I tested again the next night and it was positive. Then when my period was due I tested once again to confirm and it was very positive.
I'm 12wks 3dys pregnant and I got a very dark + 1 day late.

the day of my missed period i got a strong positive