Wiper blades should be replaced:

• Every two years or when you notice a difference in driving visibility
• Every six months to a year, or as soon as you notice a difference in driving visibility
  Explanation:

  "Wiper blades should be replaced every six months to a year or as soon as you notice a difference in driving visibility. When wiper blades no longer make proper contact with the windshield surface, they can begin to squeak, chatter, skip, smear, or streak reducing driving visibility."
• Only when they completely stop working
• Only when they are physically damaged