From my time majoring in Arch, I'd say the rule of thumb is:

"Is the culture the body came from vanished or changed to the point where no one has a personal stake in it."

So for example, vikings are long since gone. Modern northern europeans are generally a completely different culture, therefore not grave robbing. Same with Ancient Egypt, Ancient Rome, etc...

Indigenous tribes in North America and Australia for example, still very much around and still very much grave robbing (though that opinion is controversial)

Basically, if the existing culture still shows reverence to those ancestors...leave them alone. If the existing culture no longer honours them as ancestors, dig baby dig.

I guess that as long as nobody complains, it isn't grave robbing.

It's a government permit thing, not a time thing. If you go an dig up an Etruscan grave on your own it is absolutely grave robbing.

I think it's more about intent and what you do with the findings and who gains from it. If you and your team announce your plans ahead of time, document everything meticulously, deliver the pieces to a museum or archive, publish papers and deliver seminars and attend conferences on it... it's probably archaeology. The public then has at least some access to the value of your work.

If you and your associates do it all in secret, sell the artifacts to some rich asshole (esp. via a fence), and cover your tracks, that seems a lot more like grave robbing. You've stolen all the value in that case.