Bit of research for my writing...
 

I tried googling a bit, but I came up with nada results.

Here's a couple of things I'm working on - the idea for my book is an ancient civilization, expert craftsmen who unknowingly put magic into their weapons. When a group of dopplegangers, all talented in magic, take over, the people are completely defenseless, and are easily subdued.

My problem is with age and decay - does anyone know the average decay time for stone? And for skeletons? What about metal?

Try wikiing the subjects that you want to know the decay rate for. I doubt you would find anything about rock so you need to make an educated guess depending on what type of rock it is and its environment while it decays.

Well I don't know much about bone decay. But I'm pretty sure that metal stays the same unless there's something to oxidate it(if it's rustable) acid/base, extreme heat cold or erosion involved and the same goes for stone.

If there's none of these things metal and stone stay pretty much the same.
So for those we'd really need to know the environment it's in.
Same goes for bone, once the biological matter all breaks down(generally in the decades scale, depending on carrion eaters, temperature and bacteria) it's generally only affected by outside chemical reactions(ie acids) or force(erosion or having things fall on them).

*bamf!*

I believe that bone deterioration depends entirely on environment. IN a harsh arid environment, the bones would be vivibly deteriorated in a matter of weeks, due mostly to erosion.
To find the deterioration in rock strata, search for carbon-14 dating information.
Stone deteriorates mostly because of erosion.

Actually, envionmental factors are the reason that all things decay.
So depending on the environment of the planet/place then...

I think the answer is virtually forever. Things that aren't radioactive are stable, so they don't decay. Chemical actions should act pretty quickly, so if something hasn't been eaten, dissolved, or vaporaized, and it's somewhere safe, it'll probably live to be as old as the earth. So what I'm saying is that something somewhere has the odds that it will survive forever.

There are skleletons that don't stick around for very long, and then there are fossils that have survived since there wasn't life on land.

Dead organic matter decays because of microbes, and erosion.

Inorganic things (rock, bone, etc.) decay because of erosion.

So yes, it depends on the enviroment.

Dead organic matter will decay no matter where you put them.

Okay, so, like, not neccesarily in my book or anything, but let's talk about in a stone room, sealed away, with skeletons in metal armor and holding metal weapons?

Yeah, I know, you all totally didn't see it coming. Fantasy literature just does that.

So far virtually forever seems to be the common term, which surprises me.

Anyway. Masonwork would probably stick around forever, right? Good to know.

The main reason I'm asking is because I remember an episode of some show where they found someone buried under the earth, his skeleton dated to about the time of Christ. After this miracle-cloth started healing people, I figured that wasn't a reliable source of information and decided to ask some friends.

If the room is sealed, then the stones and metal would be around forever. They are not subject to the elements and would be perfectly preserved. Should the metal and bone and stone be exposed and dumped lets say a river, I am guessing it would be gone in a couple of centuries. Its all about location

Anything that doesn't break down through natural chemical reactions after death would stay preserved with a lack of carrion eaters and bacterial life.

So if the place was comletely sealed, then what'd you would have would be not so much skeletons as mummies.

Sorta like this.