I have a love-hate relationship with heels. I love to wear them, but hate when they hurt.
I'm actually ok up to about 2.5 inches. I have some shoes, and books that are about 2 inches or so and could wear them all day, they'e very comfortable. (more comfortable than flats). I also have a pair of boots, I love the look but cant wear it. I think it has a 3 inch heel, only about an inch higher than my other ones, but last time I wore them, my left knee hurt for almost 2 weeks. It was a minimum day, so I had them on for a few hours, and most of the time I spent sitting. Still, almost 2 weeks! I had to take high doses of Aleve for the pain to go away and eventually stay away.
But I can salsa dance all night in my 2.5 inch dance shoes and no pain. So maybe it has a lot do with how the shoes are made.