It really depends on how many hours the engine has on it, how well it was maintained, and how the boat is ran, if it is run at a cruise or low speed vs high speeds will all effect the over all life of the engine. Most engines these days get over 1000hours before a rebuild, and many are closer to 1500, there are always those that require it sooner as well. It really comes down to how much you want to put into that boat. Are you going to keep it a while, upgrade/sell? If you are not going to keep it long, do not repower, you never get back what you invest into it. If you are planning to keep it a while, it depends on how happy you are with the engine package. IMHO, I would run the engine until you did not feel safe with its reliability, or until you could financially justify repowering.