After watching the Grammy's and watching so many beautiful women try to walk in their heels - I felt compelled to ask this question! Well, compelled to give a simple answer.
Too high is when you are not walking properly. So many of the women I saw presenting or accepting a Grammy were walking with their posture changed, their gait changed and their hips, knees and feet not working as they should!
High heels not only put pressure on the feet but also on the knees and the back. If you are wearing heel and it does not feel like you are walking heel to toe, standing straight and having your normal stride (length and rhythm of each foot step) - the shoe you are wearing are too high!!
And by the way, platforms are not much better! Many people thing that the height of the platform negates some of the overall heel height, but I can tell you first hand that you do not have a normal walking pattern when wearing platforms. Often, walking in platforms does not absorb shock like walking in regular shoes do and back pain becomes a problem.