1. When your old ones are out of style. 2. When the old ones get really dirty. 3. When the toes get holes in them. 4. When the soles fall off. 5. When they smell so bad they could gag a maggot. 6. Other.
6. When the soles split or wear through. I go through the soles well before the toes. With sandals/thongs (flip flops to you Yanks and Poms), when the strap breaks.