How long does it take to flatten my stomach?

It depends on how much you need to lose. But if you can stick to a healthy diet and a consistent exercise routine, you should start to see results in six to eight weeks.
However, don't expect results if you eat healthy and exercise for 3 days and then go back to your bad habits. The key is to be consistent with both your diet and exercise and you will maximize your results.
Just remember there is no such thing as a fast way of flattening your stomach.
What's the best exercise to flatten the stomach? Cardio exercises are the best, not crunches. Ab crunches tone and give you definition, but cardio (jogging, aerobics, etc.) burn the fat that is hiding your abs.
You could do 500 crunches per day, but if you aren't burning your body's overall fat, you'll just be building muscle on top of our existing fat.
I'm not saying not to do crunches, just understand it's the cardio that burns the fat. Ab crunches are only effective if you are doing consistent cardio and maintaining a healthy diet. Also be sure you read my tips on proper breathing when doing ab crunches.