Since I became the dedicated bike mechanic within my group of friends, I purchased file-boxes (bulk) of cables and housing. I've only been riding for a couple years now, but I'm making it a habit to change my cables and housing at the end of the wet season here (SF Bay Area / around April).

I was just working on a friends bike a couple weeks ago. Shifting was horrible. Cables were rusted. Housing was in poor shape. Brake pads were worn down badly. Rear wheel was way out of true. Derailleurs weren't adjusted properly. Chain was caked with grease.

I asked how often the bike got a tune-up. He said that he bought the bike for his son about 20 years ago, and the only thing that had ever been replaced were tires and tubes. When I got done with it, he was blown away by the difference a tune-up can make.