When a report is made public, 'cache-expiry' is the number of seconds that the results will remain cached from the last run -- *except* if this is set to 0, in which case the cache never expires. This behavior is *very* confusing to users, who expect the cache to expire immediately if the expiry is 0, and get exactly the opposite behavior. This is a frequent support issue -- This should at least be better labeled (perhaps with a javascript warning?), if we can't find a path toward changing the behavior.
This setting seems to be gone in Master.
Oh I lie, you have to have the caching turned on. ignore that.