How are public folders archived? I have not been able to find a way to set the archival schedule on a public folder.
Public folders aren't archived, per se. What they are is "Aged." You can set the aging for a public folder in the Exchange Administrator application.
Find your server and open the Public Folder Resources object. Find the folder you want to set aging on, go into the properties, and click on the "Limits" tab. The bottom option should be the number of days you want items to "age" before they are removed from the folder.