|In fresh water, our rigging should last a long time, but as you suggest, no one knows if there is a "best before" date. I take my rig down every winter. I look carefully at all the wire, mainly searching for kinks. I found kinks in two lowers one year (someone screwing around with the masts after they were positioned on the racks - grrr!), so I replaced them all.
I give all the terminals a visual inspection for cracks, which is apparently where a failure begins. You can buy a special dye kit that will find cracks very effectively (crane people use it), but I don't know what it's called or how much it costs. Perhaps someone here will know.
C&C 27 Mk IV
PS My understanding is that x-ray ain't cheap. I gathered from a plane owner that the need to x-ray all the terminals in control wires is a major reason for the cost of small-plane maintenance.