Don,
when we had our 681, it was 100 hours and not calendar related. It is a special maintenance program that was approved for the 681 but no other turbine commanders I believe So, if we only flew 50 hours per year, then we could do the inspection every 2 years then or once we got to the 100 hours of time- could be 3 or 4 years, etc... then. It was a pretty good benefit/advantage versus the other commander turbines for sure. However, there are other time/calendar related items like the spar inspection (3 years) no matter what the hour usage is and is based solely on the calendar unless you can get a FSDO extension which we could never get with our FSDO no matter how many negative corrosion inspections we had.
So, the 681 maintenance inspection program they created for that aircraft I thought was quite unique and special for sure.
However, even though legal, running that type of plane for more than 3 to 4 years with little hours and without any type of inspection from a commander maintenance type of guy, i think maybe legal, but not the wisest way to operate that aircraft from my experience with the 681. But is just my opinion though.
One can apply this same logic to the landing gear overhaul requirement on the piston commanders, pretty close to the same type of gear setup and parts as the turbines, but
not required every 5 years to overhaul in the piston planes. Turbines they are required to overhaul every 5 years. Its wise to overhaul the piston commander landing gear at some point? my opinion is every 5 to 10 years would be my best guess if you don't want landing gear issues at some point.
I loved the 681, but i also love my 840 at least as much, if not a little bit more, especially on those 1000+ NM runs