Don't think that there is a specific time interval that defines 'short-cycling'. Depending on the system, it will have some minimum 'on' time to reach it's full efficiency. It just makes sense to run long enough to reach that point and stay there. That is why sizing a heating system for the real load is so important, not only for efficiency, but for comfort as well. Having a constant heat input that offsets the losses means more comfort rather than a hot blast followed by a cool-down. WHen you shut a system off, all of the heat you put into it is not moved into the load side, and is lost; either up the flue, or in just sitting there warming your utility room (not necessarily bad, but probably not all that useful either). This is where a modulating heating system that can adjust to the load can come in and be much more efficient.
On the cooling side, it is even more important, especially where you want to control the humidity...an oversized unit means cool, clammy space. This is where an undersized unit, running full tilt will be more comfortable since it will pass the humid air across the heat exchanger more often, and extract more moisture, lowering the humidity, and making things much more comfortable.