It depends on some points:
- how is the shoe made: Some have a curve (front to back) which is easier to wear some are harder.
- where is the heel mounted: If the heel goes straight from the back down it is harder to go in, as the traditional conave fillet (where the shoe's heel is located in the mid of your foot-heel)
- are you used to higher heeled shoes?
- are the shoes used (worn by) to you? your feet?
- does the shoe really fit you
- which shoe size you wear (3" height is a complete different story for shoe size US5 or US11)
- the underground (hard,uneven cobblestone is more difficult to go on, compared to indoor pvc-floor)
- the time period of standing/walking/sitting
In my case (rather large shoe-size):
If one or more of these points are not optimal, even a shoe with 1" can be uncomfortable
If everything I mentioned is perfect or at least tolerable:
2-3" for work/whole day is not worth mentioning,
4" Is under perfect conditions also a no-brainer but not always appropriate
5" Have tried, and can wear. But it does not look very elegant so I don't do it
- sorry for my bad english, obviously not my mother tongue, but I try my best I hope it is enough that you can understand what I mean. thx (edited several times to correct spelling)