By not connected to a load, do you mean energized but no load? If so, the worst condition for a cable is to be energized but have no load (current) through it. The cable is subjected to voltage stresses without the benefit of heating from the load current. This will shorten the life.
If the cable has been sitting in conduit or tray, de-energized and you are thinking of energizing it, I would probably do some testing such as a megger.
Cables are affected by environmental factors (especially organic insulations) by water, radiation, sunlight, heat oxidation, etc. Also the operating environment where the cable is installed can affect the life - salt spray, chemicals, etc.
WBD... What you have said does not make common sense to me, however, I am very trainable. Please provide more detail on how the life of a cable is extended because of heating caused by load current. I have never thought of heat as a benefit to the life of a cable.
Perhaps a little flippant, but in the UK, PVC with respect to cables was run over and killed be the wiring regulations (BS7671 2001). PVC cables are now to be known as thermoplastic!
I agree with what WBD says regarding the effect load as a prophylaxis against moisture, so I will not repeat any of that.
I think your last question is a request for the expected life of a cable?
There are a few dependent factors when you are trying to determine life expectancy, and it is far from an exact science. In fact I wouldn't go so far as to call it a science yet. (I know some statisticians would disagree)
As WBD mentioned, operating environment is a factor, as is the operating voltage and the insulation thickness.
Usually the cable manufacturer can give you some hints based on the application, but usually they shoot for life cycles of about 30 years for what they call "normal" conditions. (I placed "normal" in quotes becuse the manufacturer needs to define that based on the design of the cable.)
Good luck!
