When you cancel service.
Comcast and the other companies like it have a monopoly in the areas they work in. I don't see how this is right/legal. I canceled my service with Comcast and had an $80 credit on my account, even though I only paid $60 a month for my services! I now have to wait 30 days for them to process my credit and send me a check. So they make us over pay for a service and when we cancel they make us wait 30 days to issue us a credit. Great policy for Comcast. Bad Customer service and policy for the people they are gouging.