There's a lot of answers and I haven't read them to see if this sentiment is seconded, but I didn't see it from the glance down the page.
Here's a tl;dr, because my answer seems to have gotten long - makeup that is very subtle, because skin is sexy, but the girl is usually better at choosing what makeup suits them best, so it depends on the occasion.
My first thoughts when trying to answer this question is that you're asking the wrong question. However for your sake, in order to answer the question, I find that too much makeup is when it looks as though there is a layer of makeup that is caked on so much that I cannot clearly see skin.
Skin is sexy... don't hide it. So obviously I guess, if I really did know anything about makeup, I'd like the ones that advertise to be natural, even your skin tone, but not replace it. If you have serious skin problems, worry about treating the cause, such as diet, smoking, drugs etc. If your skin isn't reasonably healthy, then you're just choosing the lesser of two evils.
In addition to what I said initially, I also have this to add: As guys it's not really our place to be critical of how much makeup girls use, unless it's grotesquely over the top! It also depends on the occasion. If you are happy with the amount you have on, and can confidently say it suits the occasion, we won't want to change that, even if we dislike that amount of makeup for whatever reasons.
Don't ask the guy if it's too much, because that will invite him to be critical, when you're really in a much better position to make a decision about what you do with your skin. However, on some occasions such as being together in the comfort of your own home, I'd say absolutely no makeup should be used. Going hiking for long distances in the mountains? I don't see any point in makeup there either. I'm sure that girls would make these decisions on their own though. I've never had an experience where a girl has worn makeup at home or out hiking.
I hope this answer has been of some use to you. I respect and understand why you've asked your question, but again, although this doesn't really answer your question, but I felt as though this bit of advice answers the question you perhaps should have asked, simply because guys like me generally don't know enough about makeup to make a decision about what's too much and what's too little.