For shrouds: every 5 years is a reasonable schedule, more often for racers.

I replaced a mast tang once on an old mast (20+ years). The tang steel had obviously deformed (at the shroud/forestay shackle) and had begun to crack. I have also had the shroud anchor bar (piece under the hull lip) crack. I believe it was the result of salt water corrosion. Don't know if it is a common problem but it is worth checking once a year and also check the fiberglass lip which can wear and result in a top deck failure.