You are asking about the nature of the breastfeeding that makes marriage forbidden, and whether it must be consecutive?

Question:
You are asking about the nature of the breastfeeding that makes marriage forbidden, and whether it must be consecutive? May Allah reward you with good.

Answer:
The nature of the breastfeeding that makes marriage forbidden is that the child sucks the breast and milk flows to him, then he leaves it to move to another breast, or to breathe, or to sneeze, or for some other reason. If he sucks it, and milk flows, then he leaves the breast and returns to it and sucks it again, this is a second suckling, and so on until he completes five sucklings, whether these sucklings occur in one sitting or in several sittings. If he has five sucklings, meaning five instances of sucking, and milk flows to him in each instance, then he is considered the son of the wet nurse and of the one to whom her milk is attributed. He becomes her son and the son of her husband. Her mahram relatives become mahram to the breastfed child, and the mahram relatives of the husband to whom the milk is attributed also become mahram to the breastfed child.