Most people in the US/Canada have no information on what was done a few hundred years ago. This is archaeological evidence that before Europeans came the natives were farmers, but disease (large numbers died) and introduction of horses most of the old knowledge of farming was lost even before the old farms were force-ably taken away.

Since "conventional" farming wasn't even a thing until ~50 years ago I'd say about 60 to 80 years sets an upper bound on "long term" w.r.t. "conventional" farming.

That’s the question. If we’ve been doing it for 150 years and don’t have square kilometers of destroyed soil, what does that tell you?