Hi!
How many years does it take? a decade? I think my longest running fort in 31 was about 5 years, and the skeletons were all still there from the first two years.
That is strange. While I haven't actively played the fortress anymore, I got a save from 1058 where there are already elephant bones on my weapon traps - and I am pretty sure that it took me a few years to get those weapon traps ready.
The problem is, I never measured this, as I had too many elephant corpses there and once in a while, an over-eager dwarf would pull one of the corpses or skeletons over to my butcher's shop and clog it. (The weapon traps connect the fortress with the outside, so there is no way a dwarf could have carried the bones there by mistake from inside the butcher's shop or the stockpiles).
Just to add insult to injury here, those weapon traps are underground/inside, which should, as far as I remember, slow down decaying (^_^;;
I don't know why they didn't rot for you, but it should probably be less than 10 years until they have turned into bones.
My current fortress has not that many deaths outside the slaughtering (which I am really grateful for), so I am currently not in a position to collect data on this. But I have had rather curious foxes and I have created the first weapon-trapped entrance (in 1061 (^_^;; ), so maybe, I can collect some data.
Deathworks