I personally know of two 93 900SS's with over 40,000 miles each and a 95 900SS with over 75,000 miles. The engines are very robust. If you keep them in shape and do the regular maintenance, they should last a long while.
Around 50,000 miles, some have needed to overhaul the engine - valve guides, seats, etc...
Personally, I've done 18,000+ miles on a 94 900SS, about 30,000+ on a 95 900SP, and 20,000+ on a 98 900FE
While some do have issues - cracks, electical, etc..., once resolved, they are rock steady.