I normally replace mine every two years, cables and housings both. This year I waited three years though, as I had converted to the rather expensive XTR cables. So last Friday I had to bike home from work in low gear, as the FD cable snapped. Lesson learned.

On a related note - I have sometimes wondered if I should replace the brake cables when replacing brake pads (V-brakes), as this means having to expose cable previously clamped by the pinch bolt. Comments?