[Hummingbird] Optimal setting for the expiry period


In the Browser Caching section of Hummingbird Pro, I have the option to set a length of time for the expiry period. A note on Hummingbird tells me, "The recommended value for this file type is at least 8+ days. The longer the better!"

The options available to me run from two hours to one year, although the default seems to be a mere eight days.

What is the optimal setting for the expiry period?