interesting question. I would imagine it would be a similar concept to taking gear, even though they work totally differently on growth. Like anabolic hormones the greater the elevation, I would imagine the greater the effect on growth. Also, like anabolics, I would imagine small accumulations over time contribute to the growth of cells. Any elevation above your current baseline should contribute to muscle growth, both with IGF and anabolics. Though it may be so minute and so immeasurable. That's what lifting, eating, supplementation is though. It is there small tiny accumulations over time. Anything that is greater than what your body has adapted to, should cause a further adaptation (growth). So it would seem the greater the elevation over a greater amount of time would produce more noticeable results.
Not a direct answer. I don't think there would be a clear cut line as there never is with the body.