How long do you have to exercise to start burning fat?

Burning the calories: Any exercise is better than none in term of burning calories, including fat. For overall health, a balance of diet along with moderately intensive aerobic+wt bearing exercise 45-60min/day for 4-5x/wk would be great. Typically u will start burning sugar calories during exercise and actually burn fat when u are not actively exercising. Only u can keep yourself healthy. Good luck.