As stated above, every boat reacts differently, therefore there are no rules etched in concrete. I just finished reading an incredible book "Storm Tactics Handbook" Modern Methods of heaving to for survival in extreme conditions. The book was written by Lin and Larry Pardey, who clearly define the techniques and benefits of learning how to heave to properly. I think the most amazing part of the book is where they're heaved to in winds with sustained speeds of 80 knots (force 12) for two days, during which time the boat's turbulence actually becalmed the encroaching seas. I think this is one of those books that should be onboard at all times, especially if you are intending to do any bluewater cruising.