Never, because the basic trope of "Avatar" is as old as there have been story tellers gathered around campfires even before writing was invented.
It has different emphasis, though. We've had basically imperialist versions with the Kipling story The Man Who Would Be King, after all.
The thing is, I don't see "white guilt" in the movie at all. I see a movie about environment guilt
There's that too. But that the film has reservations about advanced cultures stepping on more primitive ones is fairly obvious, and emphasized by Cameron himself in interviews.
It's not an either-or guilt; and indeed the film here frames the problem so they're identical: In Avatar, disturbing the ecology is actually exactly the same thing as disturbing the natives since they're quite literally in tune with the system.
That said I was talking about white guilt as a concept rather than its application in the movie.