Listen to Larry...he knows!
The bottom end of these engines are nearly bulletproof...so you shouldn't decide to perform a rebuild unless it calls itself to attention...and as Larry said, condition, not mileage dictates that.
It is unfortunate, but I can attest to the fact that rarely-driven vehicles actually suffer more damage than those that are driven regularly...cars are meant to be driven, and many of the materials used in the engine will break down and deteriorate over time simply by the corrosive effect of the fluids contained within...especially the pliable materials like rubber seals, wiring insulation, and gaskets.
My 73 Bug is (was) a full-out show car, and driven only to shows for the last twelve years. Externally, it appears to be in excellent condition, but mechanically, it's a mess! Improper storage for 5+ years sealed its fate!
My tech recalls a woman with a 1990 500SL that has around 5K miles and has had three water pumps replaced! Each one failed in the same manner...the seals blew from long-term corrosion!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car