For cardio exercise, check this out:
"That translates to about 60 percent of your calories coming from carbohydrates, 25 percent from fat, and 15 percent from protein."
Above quote from "The Runner's Diet" Cool Running | The Runner's Diet
The question is, how much protein is enough? I think that depends on how much one is breaking down their muscles during exercise, so it really depends. I think the 15% figure quoted above is a little low, but I realize that getting 30% (which I had been trying to get) is probably too high. So, 20-25% protein for me seems to be what I've settled on.