How long do organisms multiply?
As long as conditions remain favourable or until the organism become sterile or until it dies.
Nova net: Until they reach the limit of their possible food supply.
Pasteurisation is a heating process that is designed to kill spoilage microorganisms and pathogens. By killing off spoilage organisms, the product will keep fresh longer since the organisms that are left first need to multiply again to be able to cause the spoilage. The process of multiplying is slowed down by keeping the milk below 5°C (since the organisms multiply slower at a low temperature).