When is it time to replace the gear-cable housing?
Most quality cable housings now have a plastic lining. This allows the cable to slide freely and prevents binding and rust formation. If you use stainless steel cables they don't rust. My local shop has plain steel cables for $2.99 or stainless for $4.99. I am a big fan of Jagwire cable sets. The mountain sets include a small plastic sleeve for the exposed sections of cable. The housing ends are sealed by a small rubber section that extends into the above mentioned plastic sleeve. There is no way for any grit to enter easily. I change the cables only when there is evidence of fraying or breakage. I have had the same housings for 3 years and they seem to work like the day I put them on.