HI THERE

There are TWO numbers suggested for TBO.

one  we are all familiar with, which is the number of hours of operation

and

the other we tend to ignore, which is the number of years in service.
     i believe that it is twelve years or some similar number of years.

i personally have ignored this "timing out of the engine"
      somewhat to my own peril.

over the course of my ownership of Large Continental  Engines,
      i have had three engine that needed overhaul due to ravages of time, as opposed to wearing out.

we all know about the "soft" cylinders" on Continental Engines;
                 That i have lived with and the regular replacement of of a cylinder  of two
but,
  the problem that causes engines to be overhauled is Lifters and Camshafts.

and,
         Lifters tend to have spalling, (the erosion of the rubbing surface of the cam follower-lifter) which leads to excessive Camshaft wear.
and ironically, this wear is less likely in an engine that has been used a lot and regularly. or to wit. a high time engine.

i am intimately familar with this problem because, recently, a cylinder removal led to examining lifters which led to having to overhaul an engine that had 850  hours TT. the engine had been installed in the late 90's

yes,
       it was out of Calender time...which i ignored when i bought the airplane.

how to prevent this problem, fly a lot; and starting the engine and running it once a month is not a good solution and may exacerbate the problem.  it is the buildups of moisture and acids in the engine over time that leads to these failures

Conversely,

CapeAir, has over 150 Continental TSIO520's that have FAA approval for a TBO of 2,500-2,700 hours, because of the high utility of the aircraft.
  (yes, that FAA)

and, the aircraft that were used for fish spotting in the northeast were going 5,000 to 6,000 hours between overhauls; those planes were often flown 10 hours a day, day in and day out.

so Calendar time matters, for me three time

peter