Mucous Plug/Second time Moms

Michaela • Aidens going to be a Big Brother to a Baby Girl Due December 21st!!
My mucus plug partially came out Sunday morning & again this Morning. Problem is I'm only 32 weeks. Any second time Moms remember how long after they went into labor?! My Dr. Doesn't seem to be that concerned but told me to call her back & go in if I had more then 6 painful contractions an hour.. ?