Mine was on about 60k when I had it replaced. It was showing signs of wear. The interweb experts say once the paint has worn through it won't be long before it fails, but this is mainly based on experience from the US where they have poorer quality oils and tend to experience greater wear. The paint had gone on mine but vRSAlex said mine probably had more life in it.
As above, for the cost of the part I think it should be considered a routine service item replaced at every major service or there abouts.