if the cable looks good at the inside of the termination eyes (where it wears on the eye bolt), it has no broken strands and isnt too tight, it should last a loooong time. a bit of chain/cable lube on it will only make it last longer.
if it clamps directly to the quadrant at the terminations, then there will be no eye to wear out.
a certain amount stretching is normal in new cable of the type used for the steering quadrant, but this will depend on how much stress its had over the years, which if its set up properly wont be much. and it can still be adjusted up a bit.
and also take into consideration that a lot of boats set at the dock most of their life... if yours was one of these the cable may be almost like new....
but if you do replace the cable, dont get it too tight... it has to have some slack in it to keep all wear to a minimum, but not quite enough to allow play in the wheel...