Master Craft recomends every 2 years if I'm correct...
It really depends on how you care for it. If you run your boat dry for even a short time you'll fry an impeller quick. My Sanger (which never sat for more than a week at a time) had close to 400 hours on its first impeller. It looked almost new when I took it out after 2 years. My last X2 went 2 years and 180 hours without any problems.
Personally I'd do it right around 2 years or 200 hours. I'm sure I'll get flamed for that, but I've never had a problem.