I think a better analogy in this case, would be if you bought a kitchen set that is advertised as "3 to 10 pieces" and you only receive 3, I'm not sure you could claim a refund, unless they mislead you in some other way.

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.