Is the Panels simple cache lifetime a minimum or maximum?
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.