This is something I get asked a lot in connection with Want Not, and my answer is always the same: It's too much when you feel deprived and monetarily there's no reason for it. (And I mean adults, here---my kids always feel deprived!) If your frugality is interfering with your enjoyment of life, it's a problem.
I agree with MaryP that it's something we rarely see 'round here. Most Americans have such an overblown sense of entitlement that they feel deprived even when they're living far beyond their means.
The husband and I had a spirited discussion about my clunker of a car the other day. He was arguing that if I want a new car, I should buy one. I was trying to explain to him that although I would love a new car, it's not important enough to me to give up the money in the bank. If I could afford a car and not buying one made me unhappy, that would be stupid. But if I can afford a car and don't feel like I need one, that's frugal!