When are plough discs worn?

[sh40]

If new discs are 18 inch in diameter when would you be changing old ones? Would he normally change when they get to 14 or 15 inch etc?

 

[davidroberts30]

If new discs are 18 inch in diameter when would you be changing old ones? Would he normally change when they get to 14 or 15 inch etc?

When you cant get them to cut half the depth of ploughing without rubbing the hub away