Apple's valuation growth is due to selling enormous quantities of products -- their P/E has roughly hovered around ~15 the entire time. If it were 20 or 30 then there'd be a lot of room to fall. But their P/E is about industry average... they just have a megaton of sales.

If iPhone/iPad sales decline precipitously (to ~2010 levels) then their share price should fall to roughly that amount [probably more, since it indicates something is terribly wrong and likely to decline further].