How long should I go on engines?

[Walter Atkinson]

An interesting issue to consider is the fact that top end problems are seldom catestrophic (rare cases are, but they are the exception) and they telegraph their impending failure for many hours if one knows how to listen to what they are telling you.

Bottom end problems are quite different. They tend not to telegraph their impending failure and they tend to be catestrophic. These are the ones to be concerned about in high-time engines. AND, there is a very big difference in a 2000 hour engine that's 4 years old and a 2000 hour engine which is 25 years since overhaul.

These issues are what make this question so hard to answer with any degree of accuracy.