There are two ideal times that are most effective in burning fat.
You achieve the best results on an empty stomach after drinking water to prevent dehydration.
When performed at this time you burn 300% more body fat than at any other time in the day because your body has no glycogen stores (stored carbs) in the system to burn. Therefore your body goes directly into the fat stores in order to get the energy necessary to complete your workout.
For about a year and a half I used to work out in the evenings. After my day of work I would be so tired that I never had enough energy to give maximum effort in my workout hence not seeing the results I would have liked.
I decided to set my alarm early and get up and exercise in the morning (and no I am not one of those morning people who bounce out of bed). I definitely recommend working out in the morning, I have so much more energy for the day, I am more motivated to eat better throughout the day, it doesn't affect your plans for the evening and most importantly I get better results.
2. After Weight Training
Another great time to exercise is after weight training. Glycogen stores have already been depleted, so once you start training you start burning fat as soon as you elevate your heart rate since it is the only fuel available.
What if you don't train at the above times?
You are just making life harder for yourself. If not done at the above times, it takes the body approx. 20-30 minutes to start burning fat. This is how long it takes to deplete glycogen stores and start burning fat. As a result your workout would take about 50 minutes- 30 minutes to give your glycogen levels depleted and 20 minute workout after that to burn fat.
So remember to be smart about your how and when you exercise to get the maximum results.