Cables have to be specified for all sorts of duty and environments. So the degradation to worry about would be UV ageing of cable insulation polymers (unlikely concern in the UK); mechanical abrasive damage to the cable insulation (doesn't sound like your scenario) and connection/disconnection cycles specification of the plug (again not exactly what you describe). I would the cable manufacturer to have modeled a worst case user using the cable every day (eg a commuter) and possibly in a much hotter climate (eg California). Personally, I wouldn't be changing my cable for a long time. For end of useful life, I'd expect to see visible ageing of the cable insulation and/or to feel unusual warmth at the cable plug, indicative of connector wear.