New shifting cables. When to replace?
 

How often do you need to replace the shifting cables. How do you know when it is time?

Every 273 days. Seriously, time is not the determining factor, nor is miles. If they are shifting OK, no worries mate. A decent cable should last the average person quite a while provided you're not riding in a monsoon al the time.

IF and WHEN the shifting gets SLOPPY, even when you've tried adjusting the barrel adjusters on the shifter/derailleur. Sometimes, it's the housing that accumulates gunk inside that prevents the cable from sliding smoothly. Washing it down with WD-40 often clears it out. Try this before replacing the cable.

I would suggest you check the cables where they are bent the most. With twist-grip, downtube and bar-end levers, this happens in the lever. No experience with STI.

Basically, all the cables that failed broke a few strands first, and eventually broke all the way. So if you see or feel broken strands near the levers, it's about time to change the cables.

If you do any significant amount of riding I would make a habit of changing all cables and housings annually. The housings come pre-lubricated and the smoother movement of the cable in the housing improves both shifting and braking.

There are no set formulae in which to gauge wear.

When it gets rusty or frayed.
Part of your pre-ride inspection should be to inspect the cables for broken strands.

Enjoy