2 levels. This is noticeable in the gnome task area mushroom gardens as when you first jump in the pigs don't move, but when you go down one level and are one level apart from them they start wandering again.
As far as how many sqms, I don't know if there is really anywhere to see this as it would be extremely difficult to gauge. I tried checking some 2 level areas around yalahar but everwhere i checked the creatures seemed to walk regardless. So maybe only certain monsters are affected by the distance or the sqm is extremely large and the areas around yalahar are potentially detecting people at depot/training dummy. One of the spots I checked was magician tower and they are 5 levels above ground and they constantly wander.
So my answer would be monsters that ARE able to be idle would be 2 floors different from them, otherwise I would just categorize them as a restless creature