Matias Vernengo has a very interesting post on the long-run and the short-run in economics. As he says, the long-run and short-run are just thought experiments (he calls them “methodological tools”). So, the long-run is an imagined period when all adjustments had taken place with which the economy is pregnant with at any given time, while the short-run is the “imperfect” period of gestation where these adjustments are taking place.
Vernengo then correctly points out that when economists actually designate periods of time — 3 months, 6 years and so on — to these periods they get into an awful mess. I totally agree. But I would go one further: the long-period is not just a thought experiment; in actuality is a metaphysical idea. Joan Robinson, who took the same position I think, used to in her later writings call it “the mysterious long-period”.
Not only does the long-period not exist and will never exist, but it is not even a real concept and any application of it to real historical time will always result in nonsensical statements. This is because historical time is non-ergodic and so as forces of adjustment move through time they cause changes that alter the course of the trajectory of the economy. In conclusion, talking about the long-period is really just a metaphysical thought experiment and the moment it can be seen to have any influence in a discussion of any real-world situation it can be known with confidence that the discussion has gone completely awry.