This is sometimes asked in jest, and sometimes as a serious question. It's a bit hard to know which leg you lean on. But in case it is an honest question I would say two major factors are:
1. The manner in which it is done and your motive. E.g. Are you taking care to preserve the grave and the buried? Are you primarily looking for things which has a monetary value? And what are potential descendants saying about you doing it?
2. The time which has passed. E.g. If it has become "forgotten" and not taken care of regularly it could maybe be seen more as a historic site rather than a grave.
I mean in the west / europe, clearing old graveyards is pretty common after a while. I looked up the legislation, basically when you buy a grave plot, you pay for the rights for a minimum of 10 years. That is separated between above and underground rights; if the above ground rights have passed, the grave stone and other above ground items are removed; if the underground rights have passed and the site has been resold, any leftover remains (bone fragments at that point) will either be excavated and put into a mass grave, or buried a bit deeper 'below' the new grave.
But a more accurate comparison would be the elite, they get interred in family crypts or under church halls for hundreds of years (or indefinitely, maybe if there's a major religious or political revolution will they get removed, destroyed, or forgotten).
The modern-day cathedrals and mausoleums may end up being the pyramids of future generations. like, 4-5000 year future generations. I'm sure they'll survive the nuclear apocalypse.
I think the way it works in france is that when you buy a graveyard plot, you buy it forever, as long as someone maintains it. If the plot isn't maintained, after a while it is deemed abandoned and reclaimed.
The one that looks like it has been designed to resist nuclear apocalypse is Lenin's mausoleum. I know I'll be dead anyway, but still, the thought that all that will be left of our civilisation is the remains of that "great ruler"...
Are you attempting to ask honest question here or are just taking a contrarian position against a view I do not hold? Are you unaware that it is fully possible to study a subject while also respecting it? It also seems to me that the questions you are asking are very black-and-white such as "And where is the limitation?" on an issue where there are many nuances.
Despite how little we know of ancient rites, I am fairly confident that being exposed in a museum or transferred to the storage room of a warehouse is far from the respect expected by the deceased when the burial was arranged, no matter how much care you take.
It’s a kind of value debate that has no definitive answer anyway. I am personally on the side that once dead, the corpse is just a stack of decaying meat which doesn’t command respect in itself. So it’s really a function of respect to the livings to whom the deceased meant something. And this wanes fairly quickly. Perhaps a century or two for a random quidam. Perhaps a few centuries for someone illustrious. But I am not religious either.
1. The manner in which it is done and your motive. E.g. Are you taking care to preserve the grave and the buried? Are you primarily looking for things which has a monetary value? And what are potential descendants saying about you doing it?
2. The time which has passed. E.g. If it has become "forgotten" and not taken care of regularly it could maybe be seen more as a historic site rather than a grave.