Well the time factor for this option also relies on triggers such as visitors viewing pages to help push along the cache rebuild. And how long a user is online.
So if you make the timeframe very short it will change very quickly.As long as the cache is rebuilt. So they may appear to come and go. It you make it extremely long the visitor will need to be there for at least that time period to appear.
It is kinda tough to set depending on how busy your site is and how long a member is there.
And giz is correct the setting in the control panel for cache is really how long a member is present. The island is rebuilt on a hard coded cache as far as I can tell.
