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The bellows should be replace at first sign of them drying out or getting brittle. You should check them at least twice a year by getting your hand in there and squeezing them and feeling the texture for dryness. I've seen them changed every other year and I've seen them not needed until 10 years. I will say it will save you money if you do it when you should do other maintenance items like changing shift cables, having U-Joints and/or couplers replaced, and anything other outdrive service.
I had all of the above mentioned on my current boat 2 years ago. So it was at the 5-6 year mark and the boat was new to me at the time so I wanted to make sure all would be good to go. Hope that helps.
Dream 'Inn III -- 2008 400 Express