How Long Does It Take for Body to Decompose?
Timeline The Basics
24-72 hours after death Internal organs begin to decompose.
3-5 days after death Body starts bloating. Blood-containing foam begins leaking from mouth and nose.
8-10 days after death Massive decomposition of organs in abdomen accumulate massive gas; body turns from green to red because of blood decomposition.
Several weeks after death Nails and teeth begin to fall.
1 month after death Body starts to become fluid.
However, much depends on the environment. Temperature, moisture, flora & fauna are ALL factors in decomposition. There are even areas where, if you laid a body out, it would end up mummifying. Really depends on what the conditions are, and you haven't supplied those, so I can't really get specific. Google body farms. General basics in this link on effecting corpses.
Most of your question didn't really make sense to me, so it might need an edit.
I was thinking that all the bones, could get cluttery. Would that be a problem?
What do you mean? Are you saying that it would not be aesthetically pleasing? And that's a problem? I am fairly certain it would be much more gruesome before it got to bones...Anyway, you can space them or not, as you please in your fictional world. They aren't just going to just lie there either, depending on the wildlife you have around. Some of it will probably get toted off and eaten. A body dump area like this will likely attract carrion birds, among other things.
What do people at national parks do with deaf bones, when they find them?
I guess you mean "dead bones" and not "deaf bones"? I am going to assume that and run with it. Depends--are the bones human or animal? If they are animal they leave them there. Plenty of critters crack open bones for the tasty marrow inside. If they are human bones, they call the police. Then the police investigate as to why a dead person is in a park and they take the body away for analysis.
I was wondering what this meant, this whole paragraph, really but especially this part:
people might be pushed to eat reach others, like in some ancient populations, or encourage acts of barbarism as happened in the Roman colosseums.
Are you talking about cannibalism? I am wondering how a sky burial would be linked to brutality and eating people?
But, then, centimeteries speak one thing to me: confinement, and detachment from what in biology's days used to be a food and natural composition cycle, and another thing: artificial ugliness.
You are certainly allowed your opinion on the matter, but this may very much depend on where you are. Here in the American South, our cemeteries are filled with trees, monuments, statues and sometimes even feature gardens. Now, Orthodox Jews don't allow preservation measures and are buried in simple wooden coffins, so they do "go back to the earth" so to speak.
That the idea of setting a body out to decompose is appealing to you leads me to believe that you have never encountered a rotting corpse. The smell is...unlike anything else. Here are some descriptions on Quora.
The people doing the sky burials often have to fend off vultures even as they prepare the corpse. The picture below is of them feeding on a human body. It might be natural, but it sure isn't pretty. Most of these bodies will end up in pieces, if the priests haven't already dismembered them. Do keep in mind that rats, vultures, and other scavengers will be hanging out, waiting for a taste of your dead people, especially if there are a lot of them. This can lead to a spread of disease, if there is a population nearby--and could even lead to that during a funeral. Because these bugs and pests do carry all sorts of things. That's mostly WHY we have a practice of burial. Sure, there's the religious aspect, but if you have a decent population, you're going to get a lot of dead people. Having a space dedicated for them where they 1) won't attract pests above ground & 2) where they won't contaminate the water supply is a good thing.
You can always just place them in the dump outside of town, since refuse will already be attracting scavengers in that case. However, since people want to honor the memory of those who have died, I am sure that would be seen as disrespectful.