Your mucus plug can come out as one big "blob" or it can come out piece by piece. Mine came out over about three days, but was very obviously not just discharge - it was gloopy like snot (sorry, iots the only description I can think of) and stringy and kinda greenishy in colour. It can happen hours, days or even two - three weeks before you go into labour, so it;s not really a reliable indicator of how close to labour you are x
My mucus plug came out all at once, i just looked like a slime ball when i peed, i had no idea what it was and my nurse told me... That was like 2 wks before i had my son... I dont know if it could come out little by little tho, mine didnt