I'll be the lone voice of dissent here- I replace the rear derailleur cable at least once a year.I also replace the rear housing loop at this time. I find all other cables/housings to be much less critical and only replace them as needed.

The old rear cable becomes the new front cable. The most weathered part of a rear derailleur/brake cable gets cut off,and is fine for front use.

Note that I use Campy Ergo levers and front derailleur shifting is much more verastile and less finicky than Shimano STI front shifting, which I abhor and refuse to use.

dan