Dan333SP

This-

You can usually notice when the cable starts to fray because you feel a bit of slop in the shifting, but sometimes it can go from feeling frayed to outright failure within a few shifts, and sometimes it can last for a few days in that condition. Once it does snap in the lever body, it's a mess to remove the remains of the cable without leaving any metal strands hiding in the shifter internals.

Shift cables are so cheap that I see no reason to not just change it proactively at regular intervals.

Drew Eckhardt 

Buy a bunch of cables - like 10.

Figure out what sort of life you get with your shifting habits, gear levers, and those cables. That could be 1,000 - 20,000 miles.

Replace before they break because getting jammed cable bits out is a PITA and finishing rides with one cog is less fun.

You want to use the same batch of cables because different metalurgy, finishing, etc. can radically change life and you don't want surprises. My last batch of Campagnolo cables were good for at least 2,000 miles. The current ones fail closer to 1,000. I miss the old levers without the tight 90 degree guide that halved cable life, although the new ones are more comfortable.

Notice when shifting gets sluggish due to a groove wearing in the housing lining which pinches the ables and the rear loop getting dirty. Round down to the preceding cable replacement so you take things apart less.