My boat lives on the beach on the ocean and I replace bridle, forstay and shrouds every other year. The forstay pigtail up top takes most of the load and wear so I replace that every year. I had a shroud break last year that was only a year and a half old but I believe it was made wrong. If you live on a lake in fresh water, you can probably double that. The salt water really does a number on the rigging.

Any strands broken indicate significant corrosion and should be replaced IMMEDIATELY.