| Fil,
I took this to a couple of my professors because I wanted to make sure I had a correct answer. The concesus was that if you start to burn protein or muscle that you are doing something wrong--you're more than likely in starvation mode. Interestingly, apparently even after two hours of heavy training you still shouldn't be burning protein. Both brought up Lance Armstrong that until his last couple days of continuous riding was he burning muscle.
They also said unless you have yourself hooked up at the university, there's not really a breakdown because so much depends on how much fat you're carrying, how high of glycogen stores you have, etc. However, they did say if you wanted to go with a semi-quantifiable number, after 20 minutes of aerobic activity, you'd be burning both carbs and fat. And then after 40 minutes you're more into fat burning than you are carb burning.
There was the comment that when you engage in continuous training you will need a higher consumption of carbs and that to properly kick start fat metabolism you do need a higher level of carbs.
Hope this helps.  | 
