I can't say there is a specific benchmark for changing them, but my general rule of thumb is that I change my cap/rotor every season. As for sparkplug cables well that is relative to how good they are to begin with, are they burned from exhaust manifolds etc. I would say I would change them at least every 3 years but no more than 5 or 6. Again it all depends on how good they are from the beginning, how much usage and how well they are taken care of.
These are items that I believe are best to change whenever in
doubt. Not hard to do and it's good insurance for a fun day of boating.