Diamond wires corrode at the same rate as any other rigging, perhaps even faster since they are outside all the time when your other rigging is away.
Additionally, you should check the brass block fitting inside the mast for corrosion.
I've seen wires fail plenty of time that showed no indication of breaking, though they were all several years old.
Sailing in saltwater for our 5 month season I would replace my rigging every 3 years.