I didn't start to lose mine until I was in labor. Not everyone looses theirs before labor. And it can take a few weeks or a few hours until labor starts.
I've been losing my plug a little at a time since I was 32 weeks. I'm 37 weeks now. Dr was never concerned because it can rebuild itself unless you've started dilating. I'm at 3cm now and losing more and more of it. I've been told Labor can be anywhere from hours to weeks away.