I think I just "lost" my mucous plug. After how long will labour start? This is my first baby.

It's depends. When the mucus plug dislodges from your cervix it doesn't necessarily mean that you're in labour. If this is your first baby, Labor may begin soon after the mucus plug is discharged or one to two weeks later. It could also be a sign that you're in early labour. Early labour, where you may feel mild contractions, tummy aches, cramps or backache, can last for hours or days. Usually you will need to prepare to deliver when it's accompanied with frequent contractions in regular interval or if your water bag burst.