Well, there are cast iron hulled tall ships from the 1800's still afloat. Is that long enough for you?
You've not doubt heard the story about George Washiington's hatchet, "the handle has been replaced three times and the head twice, but it's still the original hatchet". Steel boats are like that. Keep a coat of paint on them so they don't rust, and they can and will last literally forever. Bang a hole in them, and you can always weld in a new bit of metal, welding fuses it seamlessly so anything can be replaced and made new. Sandblast it or abrade it too thin, or let galvanic action eat it, and likewise you can always replace a place or add some on.
With proper maintenance? Forever. Same thing on a wooden boat, you just need new wood from time to time.