about cache notification poll frequency

Question

I originally asked this question:

http://social.msdn.microsoft.com/Forums/en/velocity/thread/6987600c-6c02-4f07-803b-90b7959268c6

and stumbled across this one too: 

http://social.msdn.microsoft.com/Forums/en-US/velocity/thread/f70c7e3a-37fc-4b9c-8953-18dd005cdfbd

If I do not want to spawn a separate factory and manage a second DataCache instance.. how low can I possible set the poll interval without impacting performance in a negative way? Are there any figures I can calculate with? We are talking about a "small" cluster.

Also.. given the fact that your ASP application frontend #1 is updating the cache (both local and cluster) and all consecutive requests will go to frontend #1 as well (sticky sessions) there's not much pressure on updating the local cache on frontend #2, right? I.e. if a user changes data he will actually see the changed data immediately... only if he was able to switch  to another frontend (delete cookies...) he might get "old, staled" data (until the notification kicks in). Right? :)