Even on the schedule suggested by Jake, you should still take a good look every once in a while. I had the factory shroud on the 4.3 fail after only a year and a half. They were vinyl coated and the failure was down in the swage, so it wasn't somewhere I could easily see - I went up a size (diameter) when I replaced it and did new wires all the way around.

Bottom line - if you already see a broken strand, that whole wire is done. Don't just replace the one, replace them all. Nothing is more disappointing than travelling eight hours to a regatta, losing the rig before the first race, dealing with getting the mast back on the boat, pulling the main off and rolling it up, begging some fishermen for a tow, then sitting on the beach waiting for everyone to get back.