The standard has always been whether you have any wiggle when firmly trying to move the shaft outside the boat. If it doesn't wiggle, you don't need to replace it, so it is said. I really will never know why my bearing had to be replaced the first time. The boat had been on an inland lake for 2 years and then I had it shipped by truck to Long Island Sound. In my first yard period after moving, it was clear that it needed to be replaced. Maybe things got out of alignment in shipping or maybe when the boat was initially trucked from the dealer in Chicago to my lake.
However, when I had the engine mounts replaced and an alignment done professionally, I decided Deaton's should also just replace the cutless bearing. They have a finite life, but I don't know what it really is under ideal conditions. I will say that if I were going on a long trip and had a couple of thousand hours on the bearing, I would want it replaced. But, I have always been a believer in preventative maintenance and replacement.