So, order of a hundred years?
Note : you asked about "bones", not fossils. The process of turning a bone (any tissue, really, but most often a bone or a tooth, for a vertebrate) into a fossil is a subject of it's own, stretching in effects from forensic science, through archæology and into regular palæontology Look up "taphonomy".
A 100 year lifetime isn't at all unreasonable for a structure. No structure is eternal (though the Pyramids are making a decent attempt - they'll probably not make it beyond their half-million).
I was driving over the new Forth road bridge recently, eyeballing the 140-odd year old riveted cast iron of the Rail Bridge, and the old road suspension bridge (which made it past it's 50 year design life but didn't make it's century because of increases in vehicle loads and counts leading to accelerating wear rates). I wonder how they're going to bring the old road bridge down? Dismantling, or dynamite? There are enough ships using that channel that dynamite has a *lot* of difficulties.