Is the Panels simple cache lifetime a minimum or maximum? In Panels, one can configure caching for blocks/content panes/etc.  The built-in method is Simple Cache, for which two settings can be configured: Lifetime and Granularity.

For Lifetime, one can choose a value from 15 sec to 7 days.

Is this lifetime a minimum or a maximum?  In other words, if I set the Lifetime to 7 days for a content pane, will that content pane be cached for 7 days no matter what?  Or will it be cached for 7 days unless it is updated?
Simple cache, is unfortunately not very clever, hence the name "Simple".

A 7 day cache will be cleared when the first request past the 7 day mark occurs, so it is the minimum lifetime of the cache.

The reason that it isn't smarter than that, is probably that content panes can come in a great deal of forms, and it's not trivial to implement a generic solution to knowing "When something was updated".

Continuing on that, I've implemented Panels cache which attempts to be smarter, but is limited to Content panes based on an entity context. It considers the cache lifetime to be infinitely long, but clears it when the entity that is the context is updated.

It's not terribly well tested, but it was written for a paying client that had it running in production without issues or complaints at least for the months I worked with them.