Example: How do I prevent my refresh token from expiring?

Created by: retorquere

I've put together a minimum working example at https://gist.github.com/anonymous/efabe5c1122d756cd0b3. I'm getting the initial token OK, I can get a token for the initial code, and I save the token hash to a file to restore later. I can run this OK for a few times, but after an hour or so the token expires. I can see an 'expires_at' in the token hash, so in some sense it is to be expected, but how can I claim a long-lived or even "until revoked" token?