Shorter exercise (1 hr) don't eat. Longer exercise, eat something. And then throw in there how you feel when you do that and modify from there. "Science" has gone back and forth on this but I think the general consensus is that for the purposes of weight loss (fat body %, yeah, we know) it is best to exercise without eating. You then have a time frame to take start fueling the body to take the most advantage of what you just did. This again is debated over and over. I personally get some easily digestible protein in my body anywhere from 30-45 minutes after I stop.