SCs can go 200k or even closer to 300k without major engine work with the right circumstances, and this includes the chains. If the bottom head studs get corroded, bets are off, but you get my point.
The normal routine I've seen is when you do a rebuild for another reason (or change cams, say) you check the chains and gears and replace them together if need be. The chains are pretty bulletproof and you don't see them getting replaced at a standard interval.
I'm not sure offhand what the chain pitch is but they make a chain stretch tester for 1/2" pitch bicycle chains, if something similar is available for our chains it might be a good idea to check. Here's what the bike chain checker looks like, it has pins on each end and the one on the "dial" end is eccentric:
