not necessarily bro, you are always burning fat (unless doing extremely high intensity work). Think of it as a sliding scale, fat is the slowest form of energy and takes the body the longest to convert into glucose and then into ATP. It uses fat during rest mostly, but think of it as a sliding scale, you be burning 90% Fat and 10% glucose sitting here typing, but jogging you may be burning 70% glucose that comes from gylcogen stores and 30% fat.
high intensity work over time will alter this ratio over time and you will be able to metabolize fat at higher intensities. While that same jog a year ago metabolized on 30% fat, after higher intensity training, it could be 45% fat. (probably not that high though)
i dont know where the 20 minute thing has come from, i would that would be the marker for seeing any cardiovascular benefits, not fat weight loss benefits.