On my Bike Friday I have been changing right (rear) shifter cables every 12 months to make sure they would not break during a ride.
For information, I was riding about 9,000-10,000 km per year and had two incidents where I lost the use of the rear deraillleur during rides because of a snapped cable. Once it happened after about 20 km along the Tamagawa, the other time it was only 1 km after I left home. In either case I still had three gears because of my triple crank and easily made it back to the base. Both incidents had happened after about 18 months of use.
My Bike Friday shifter is still a 10-speed Tiagra 4600 with the cable exiting to the side, not under the bar tape. This type of routing makes it easier to remove the handle bars for packing the folding bike in a suitcase for travel.
Newer 10-speed and all 11-speed groups by Shimano use under-the-tape routing for shifter cables, which means the cable doesn’t run straight from where its anchored through the shifter, but needs to take a narrow 90 degree turn under the rubber hoods before entering the outer cable under the bar tape. This potentially puts more stress on the cable, even with the fancy PTFE (teflon) polymer coating on Shimano’s most expensive cables. I recall there were many reports of frequent cable failures on Dura Ace 9000 shifters that appear related to this bend, which were corrected on DA 9001.
Anyway, with my 11 speed ST-RS685 shifters with polymer-coated cables I was also planning on annual cable swaps. Twelve months and three weeks later, on a 180 km ride in west Izu I found that on a descent I could not upshift into the top 4 gears. If I downshifted and upshifted again I could sometimes coax the derailleur to get the chain onto the second smallest sprocket, but that was about as far as it would go. I finished the ride with no major problem (in west Izu as long as you have climbing gears to climb and can freewheel down descents, you really don’t have to worry much about speed on the flat stretches because there hardly are any!).
As expected, the cause was the shifter cable, which was totally frazzled and only seemed to hang together by one strand of wire or two. It seems like 12 months is right on the limit for this kind of cable for the kind of riding I do. Perhaps I will need to change it more frequently to avoid issues on the road, like in the middle of a 400 km brevet or on a mountain down in Boso.
What kind of mileage are you getting out of shifter cables on your bike?
This is one area where I’d say there would be a clear advantage for electronic shifting. There will be no mechanical cable wear and even if you run low on battery, at least with Shimano Di2 the front derailleur will stop working before the rear derailleur does, which gives you quite a bit of warning.