Life of cables
The only bad things that happen to used cables are corrosion, dirt and fatigue. The first two increase friction, so the brakes become less effective. Metal fatigue causes individual wires to snap, one by one until the cable is too weak for safety and liable to break instead of brake!
After 20 years (or less) you’ll surely need new casings, since accumulated wear, debris and grit are hard to remove from inside them. But if you’ve taken the cables out and found they clean up well with not a single wire broken, they’re safe to keep using until you do find a break. The cable will have about 19 wires in total and probably won’t fail until several have snapped. That’s your safety margin for what may happen between inspections. Once a year should be often enough, provided cables are replaced at the first sign of fatigue.
Dismantle the cable completely for visual inspection. Flex the cable to open up any breaks so you can see them and pay especially close attention to the first few centimetres near the nipple.
This was first published in the December 2013 / January 2014 edition of Cycle magazine.