As a paying customer, how much downtime until they have to refund me?
Developer's time viewed as unlimited! Because the API has no SLA and can go down for any period of time...
This is because of the terms of service you agree to when you first signed up to OpenAI, which has "WE DO NOT WARRANT THAT THE SERVICES WILL BE UNINTERRUPTED, ACCURATE OR ERROR FREE" in it, in the "Disclaimer of Warranties" section. https://openai.com/policies/terms-of-use
They upfront tell you that it won't be uninterrupted, so I'm guessing you have little legal rights here for some hours of downtime each month. If it was unavailable for a month or more, things would obviously change.