JesterMagic - 7:06 pm on Apr 20, 2013 (gmt 0)
I find breadcrumb rich snippets once discovered seem to stay.
Review rich snippets for the most part seem to stay as well.
Review aggregate and author rich snippets seem to appear and disappear a lot though (like one month there and the next month gone, repeat). These are pages that are usually linked more to (both externally and internally) using different link text so I have wondered if that every plays into it.