eh in some fields that is almost impossible to guess at. Service industry can generally bank on certain times of year for things, holidays, season changes, etc. Other industries such as mine for example depends on a slew of unpredictable occurrences. If I was in a salaried position I would never expect a time frame of my hours. One day I can do nothing all day, I sit at home on call sometimes because im 2 minutes from the plant, other times I stay and sit in my office doing paper work and playing gameboy. Sometimes I actually have to work.
There is no way to predict when something in the plant will fault, a drop of oil can spill off anytime on to any proxy anywhere in the plant, a robot could fault out at any time. Its just to unpredictable to give a time line.
It is a noble expectation but generally unrealistic in a great many work industries.