I found my limit recently and it is a 5″ heel. I have a bit of arthritis from some old college injuries that limit the range of motion in my right ankle and I no longer walk when they are that high. So, 12 cm or 4.7 in is my current limit. I also have several 10 cm (3.9 in) heels and wear them quite frequently. I found that my best pair for walking are my black boot with a 10 cm heel. I literally walked a few miles in them while in Vegas.
As of now all the heels a stilettos. However, my girlfriend says I need to get a pair of wedges for the pool. I guess as long as they are about 4-5in I can consider them.