Less than 12 hours. But that's only because I was having bad contractions and went to the hospital and they checked my cervix and I wasn't dilated any so I got sent home and lost my whole plug then went back for them to tell me I was 6 centimeters.

Lost my plug/bloody show at 9am contractions started , my water broke at 11am & I was admitted by 1pm !

Early labor began for me about 12 hours after losing my mucus plug (but mine was bloody show, as well).