As far as when to replace, or at least check, after 5 1/2 seasons in salt water (6 month season, on a mooring, not flushed till the end of season, filled with antifreeze over the winter) I found that one manifold (these are the one piece units, no separate riser) was clear in the 4 water outlets, the other had clogged on one outlet. I was able to clean out the old ones, but still installed the new set to be on the safe side. I think that 5 years, you should at least pull off the risers to check. I saw no sign of cooling water leaking into the exhaust gas areas. The exhaust ports on the engine had no rust on them at all, no rust on the exhaust valve stems, which is what you want to see....