It is checked every time you come back to the site. also, I notice goes up 30 days prior to it expiring. However, we still occasionally get inquiries.
I suspect that the authorization is revoked as soon as you change your eBay password, but I’m not sure. I have been meaning to do an experiment.
A way to demonstrate that it is active and working is to add an item to your Watch list on eBay and use the 'Import Watch list' button on the Pending page. I will give you a free year of Unlimited if you can help me test the above, namely, whether changing your eBay password kills the auth.
bidslammer