How Long Should The Feeding Bottle Be Sterilized?

Hello, I know a young couple who leave their babies bottles with old milk in them for one, two, or three days before washing them. Does more bacteria form in these plastic bottles (nipples and all, they don t rinse the milk out, they just let it sit in the bottles) that may not be gotten rid of in a sterilizer for five or 10 minutes. Thank you. Harriett