FBinNY 

There are industrial standards that speak of minimum drum and pulley diameter in relation to cable gauge. As the pulley diameter shrinks the life expectancy before metal fatigue sets in goes down. Most shift levers are near or below the standard design ratio because of space and other limitations.

Modern gear wires are also more prone to metal fatigue form flexing on the lever drum than older wires were because most today are die drawn, or squeezed tighter to be smoother on the outside. This trade off for smoother shifting has a significant cost in fatigue life.

Overall gear cable life depends on where you live and how you ride. It's measured not in days or miles but in the number of shift cycles. Interestingly, people in fairly flat areas may have the shortest life because they might be shifting back and forth between two or three gears, which means that they're always flexing the wire in the same place.

BTW- the wire will try to tell you when it's getting ready to let go. Near the end the wire may stiffen, or have a single strand or two fail changing shift performance. If you suddenly find yourself constantly fiddling with the cable adjuster, and it's been a long time since you replaced the wires, do so now before your 10 speed becomes a 2 speed.