The Folder Properties dialog (on the menu shown with alt-click on a folder icon) has a number of tabs, but the Mail Expiry Properties dialog is separate. I think it would make sense to remove the separate dialog and make it a tab on Folder Properties. I set the expiry properties sufficiently infrequently that I'm usually caught out by this, and initially go hunting for the settings in the folder properties.
Excellent suggestion, but this is a contentious issue due to usability concerns. Please vote for and add any comments to the duplicate bug. *** This bug has been marked as a duplicate of bug 115611 ***