When doing cardio, we need to consider something known as substrate utilization, which is the process of using different sources of fuel for energy. During moderate intensity cardio (65-75% of max HR), roughly 50% of substrate utilization is from fats, where triglycerides (stored fats) are broken back down into 3 fatty acids and 1 glycerol molecule. The glycerol molecule then undergoes a series of chemical reactions known as beta oxidation, where it is converted to glucose and burned for energy. The remaining 50% comes from carbohydrates, with 80% of that being muscle glycogen. As exercise intensity increases, fat oxidation decreases. So by going real hard, you will burn more total calories, however less will be from fat.
The best time to do this cardio is roughly 2 hours after a normal meal with low GI carbs and protein. When waking, your body is in a catabolic state. Glycogen is depleted, so cortisol levels are high to break down muscle into glucose so you must eat asap, exercise will drastically increase cortisol. A study by Sotsky MJ, Shilo S, Shamoon H. (1989) tested the effects of moderate intensity cycling (60% of max HR) in normal and low glucose (empty stomach) conditions. In the normal glucose conditions, cortisol did not increase significantly, while in the low glucose conditions, cortisol increased by 400%.
As for cardio after a workout, it is been shown that you have a 30 minute time frame after a workout where you must ingest nutrients to
replenish muscle glycogen, or else glycogen uptake is severely restricted, and other benefits are eliminated such as the increased protein synthesis, GH/test increases, etc. The only thing you should be worried about after a workout is getting a proper shake and meal.
So to conclude, the best cardio is at 65%-75% of max HR. The longer you do this, the more you will oxidize fats. I would reccomend this cardio at least 2 times a week at around 45 mins at a time on bulks, and would up it to 4-5 times on a cut.
