In economics we often say that decisions take at least 5 years to take full effect and to manifest their full impact. The 5 years number is not a "magic number", but an average meant to encompass the cycles of any economic decision.
Short term happens in up to 2 years, in which implementation and first results can be observed. It is not that everything said short term will simply be come and gone up to two years, but all the relevant aspects for short term impact should happen in that time.
Medium term is between 2 and 4 years. This is the time more complex policy and projects can start manifesting their goals.
Long term is over 5 years. Those are the impacts that are mostly sustained, more concrete, and the structural changes. This number comes from the measurement of cycles I have mentioned before. Changes merely financial follow the financial cycle, as accounting and auditing measures are observed mostly in medium term, but structural economic changes remain and influence through the long term.
I might add, it is an "exaggerated" idea that "in the long term we are all dead", and something that would refer to the war time economics. You will not solve or mitigate problems caused by war and conflict just focusing on long term, but long term can bring wars and conflict you are not aiming to. It is sure to bring those you are aiming to, however.