there's a guy on the Ducati_SS yahoo group who has a 900SS with 122,000+ miles on it (yeah miles not kmh). Lots have 40K+ with no probs but similarly most with 40k+ talk about new rings and engine refresh.
Register with yahoo groups. Then join >
http://groups.yahoo.com/group/ducati_ss and find lots of information on anything SS wise, particularly engines, pipes and slippers.