MANY things factor into the answer. Salt water, fresh water, was it maintained as it should be, kid owned or adult owned. But I'm guessing with 550 hrs you are likely buying a rental ski. A ski used in fresh water and maintained well can easily get 10 years of use and never need major work. I have never owned a ski less than 11 years and I have yet to replace anything but spark plugs and one oil container that got a hole rubbed in it.