How long will it take for me to get a flat stomach when doing crunches?

Can't predict that. : Your total body fat- plays a major role here. Exercise alone will not normally give you flat abs. For a flat abdomen eat healthfully ; be physically active. Calories burned must > calories consumed. To tighten ab's do crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge ; “bicycling” exercise on your back. Engage in regular aerobic exercise