In aircraft we use hours of usage as the main measure for maintenance rather than flight miles or time after last installation/rework. If needed, the maintenance intervals in hours are increased or decreased to suit the level of severity of typical usage for that aircraft.
I've often thought of using running time on the Z to measure oil life rather than miles--highway miles are easy on the car but make it look like maintenance is needed right away if you use that figure. On months, you may want to ask Bob the oil guy or another expert. I would figure a year or two wouldn't hurt the fluid, but that's a pure uneducated guess.
The goal is to change your oil and filter out before the oil becomes ineffective as an engine lubricant/etc. The maintenance schedules given to consumers are conservative because people don't follow them terribly closely and they poeple doing the recommendation have to assume a usage spectrum that covers everybody.
If you really want to know a hard and fast rule, look into what breaks down, get an oil analysis of your used oil, adjust your cycle, and then get a second oil analysis to confirm you're good to go.
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