that depends... when you reach the glycolytic threshold you have no choice but to burn carbs, but that point can be moved quite a bit. At this point, if you have no glycogen, you have to convert amino acids to glucose, or stop AFAIK. I don't know all the specifics, but there are a lot of variables. How used is your body to using fat for fuel? Doing a ketogenic diet seems to push the threshold higher, and you can train for it as well. Kiefer has mentioned this in some of the podcasts.