Yeah well even S&S says not to expect long life from their big engines...........I figure they know alittle about what they are doing.
All depends on the build, and how good you are with maintenance. Reaper's engine started life as a stock 88ci, and was built into a 98ci, 10.5:1 compression, head work, cams, etc... pushing ~100hp/~120 ft-lbs at the rear wheel. She has 294,000 kms (almost 183,000 miles) and 15 years on her, and has been rock solid reliable, from bopping around town, to multi-week/multi-thousand mile road trips.
I just rebuilt her top end after almost 100K miles on the last rebuild, and am looking forward to the next 100K+ miles before another top end (top end rebuilds are not a sign of a problem with the engine. Just a thing that needs to be done on high mileage air-cooled engines).