How long does it take you to get rid of your belly by doing 10 sit ups per day?

very difficult to say. sit ups are a very good choice but i think you should mix it up with cardio exercise also. i.e brisk walking/jogging/running/swimming/playing football/hockey/tennis whatever you like. I guarantee your belly will lose fat . You can also perform some abdominal exercises like crunches etc. Look at the website shapefit.com or scoobysworkshop.com It will help you a lot