A blocked or clogged milk duct will go away within 24 to 48 hours.
If the blocked or clogged milk duct hasn't cleared up or gone away within 48 hours then you might need some medical treatment to unclog or unblock the clogged milk duct.
When breastfeeding you can usually tell if you have a clogged or blocked milk duct because the baby may be fussy when breastfeeding on that side of the breast due to the slow or not flowing at all.
If the baby is fussy on that side then you might need to try the other side to see if the other side of the breast milk duct is not clogged or blocked.
It's a good idea to have some breast milk already pumped and stored or use some carnation milk or formula to feed the baby in the event you have some clogged or blocked milk ducts.
But usually when I had a clogged milk duct it cleared up and went away on it's own within 24 hours.