I change mine every spring. They still are pretty clean, but I'm not going to gamble that they might get gunked up offshore one day. Preventative maintenance is just that...preventative.

On a side note, if I still used yamalube, I'd be changing them 2 or 3 times a year. Now that I run a synthetic blend, once a year is more than enough. Alot depends on how many hours you run the motor each year and what oil you use. Some oils burn much cleaner than others.