To receive credit, how long must an outage be?
Generally speaking, if you ask for a credit, they'll give you one. There isn't a policy about how long the service interruption lasts.
@Nerdburg is correct. If you have any concerns related to service interruptions and your account, I'd be more than happy to assist you. Please send me a private message by clicking on my name, "ComcastChe" then click private message me. In the body of this message please verify your first and last name, street address including city state and zip code, and the account number or phone number associated with your services.