Dangerous Terrain?

It all depends on your computer's processing power. So the answer is, sorry to say, as high as the game can handle. Terrains are pretty well optimized, so they can go pretty high, but getting your game to run well is mostly trial and error. My guess is that it will get much higher than you would ever need for a game before any errors develop. You may experience some GPU lag when you are looking down from the top of the mountain, but that could be fixed with some fog and clip planes (Mountain peaks are generally foggy anyway, aren't they?) adjusted in scripts based on height of the player. If you wanted to get really fancy you could create cloud and building zones to alter the for amount further. But the straight answer is: There is none. If your game crashes, you still have it in unity and can fix it. I don't know why one would need a terrain that was so high it would cause the game to crash, though.