Definitely more than three inches.
this is not news
New Study Shows That High Heels Increase Your Chances of Falling and Hurting Yourself
A recent study by shoe company MBT (which may not be an entirely reliable source on the matter, since they specialize in comfy, funny-looking sneakers) shows that people who wear heels taller than three inches suffer from a 40 percent higher chance of falling and sustaining injury. Meanwhile, they increase pressure to the ball of your foot by up to seven times. We have additional "news": Women have already discovered this through their own personal experiences, and will continue to wear heels and fall on their faces. [Guardian UK]
