Originally Posted by Kai Winters

Changing cables should be a maintenance item...perhaps every year or longer depending on how much you ride and also how often you are changing gears...
You should also use a 'coated' cable...'slick whips', etc. as the coating allows the cable to 'slide' better than a basic cable...they are more costly but they do work much better and likely last longer.
I ride around 10k miles annually and when using mech shifters I changed them every winter whether they needed it or not. Having had to dig the broken remains of cables out of shifters is a real pain in the arse...customers bikes, not mine fortunately.