Forget it! You will not be losing any belly fat with just sit ups.
If this was remotely possible, all you would see it on The Biggest Loser would be contestants doing sit ups non-stop and plenty of product placements for As-Seen-on-TV abdominal machine gimmicks for $49.95
Sit ups will however help you develop muscle under the layers of fat (if you happen to be overweight).
Your goal however should be to lose overall body weight, and fat deposit with total body workouts.