I started wearing 4" heels almost 20 years ago. It was hard at first, but now anything up to a 4" heel (stilleto or block) feels as natural as flats. I even find most 5" heels to be comfortable for long wearing and walking, however at this height my gait is obviously slowed and my steps are noticably shortened. I've got exactly one pair of 5.5" boots that are quite comfortable for long durations.
These are good:
http://i132.photobucket.com/albums/q...nee3cropav.jpg
Above that, things get pretty iffy. I can do 6" for a limited time and distance, and higher gets painful. It's funny how a one-half inch increase can make a big difference in the higher heels.
These hurt:
http://i132.photobucket.com/albums/q...-crop3-web.jpg
For the record, I usually wear size 12 (US Womens). I would think that a person's maximum heel height would be somewhat dependent on foot size..