Ya, I'd say 2-3 hours before make it your last meal prior to the cardio, and low carb, if you have it, use r-ala with it, to make sure your insulin/blood glucose levels are low. Perhaps even 10 mins before the cardio if you can tolerate it.
And here's an idea, you could do an intense set of 20 squats right before hand, that way, if there's glycogen in the liver, should help deplete it, then cardio and you'll be burning fat sooner.
then right when complete, slam down some protein/fat, to prevent catabolism.