I put 550 hours on the motors that were in my old boat. They were 502's, 550 hp, merlin heads and solid flat tappet camshafts with stock GM rotating assemblies..... changed valves, springs, guides, milled heads etc. etc. at 250 hours... 90% of its life was spent between 3-4k rpms with WOT bursts every once in a while but for no more than a minute at a time.
525's have better parts than what I had, if treated properly I don't see why they would last as long if not longer with reg. maint.
PS, I didn't have leaky CMI's on those motors.... Stainless Marine all the way!