FLAT STOMACH FAST!
Good rule of thumb try to do 30 minutes of cardio daily with some light strength training 3-5 days a week.
But it is important that you listen to your body. Rest when your body needs it. And pay attention to what foods bloat your stomach.
I love HIIT (High Intensity Interval Training) exercise because they give you the combo of cardio with light strength training in one workout. Many HIIT exercise (ie Tabatas) last anywhere from 4-30 minutes.
I built my HIIT exercises (Core Blast, Sexy Booty, 20x10) from exercises I found here and at ab-workouts-guide.com If you like em try Cardio Fight Club HIIT, Core Abs & Stretches, MAX Tabata Test, or Multi-Plank Challenge they're fun and challenging.
Good luck and God Bless!