When does an engine needs an overhaul?

When does an engine needs an overhaul? There are several indications
that the engine is comming to the end of its life. But what do peolple
really look for to decide an engine is to go to overhaul.

Low oil pressure, High oil temperature, Low cylinder compression, Loss
of power. Which is the factor that maters most. For example low
cylinder compression can easily be fixed replacing the cylinder not
necessarilly an overhaul.

Of course I am talking about part 91 operation. I have heard people
that run engines 1000 hrs and far more than ten years past TBO.

For those of you who decided to overhaul your engine. What made you
spend the big buck?