How long does my run need to lose weight?

When you run to lose weight, trying to make every race as efficient as possible, how long does it take to last? Longer, slower races and shorter, faster races have their place in your training program, whatever your goals; changing the types and speeds of your races helps you avoid injuries and become a more balanced and resilient athlete. But should you reach a minimum threshold if you are trying to lose weight? And how do you know what it is?