Question says it all. I couldn't find anywhere in the API that states what happens when it expires. Do you have to open that "Works with Nest" webpage again and get a new authorization code? Or, using the same authorization code, just get a new token? And how will Nest indicate the token has expired?
Expiration date comes back in the OAuth response and is set to 10 years in the future, which is practically forever. That gives Nest quite a few years to come up with a refresh or expiration story.