To be honest, the answer to this question is going to be different for most people. It all comes down to the current state of your abdominal region. If you have a lot of body fat then you will have to spend more time on it then someone who already has a six-pack. There is no reason to overexert yourself, though, as you should only do what your body allows you to do. If you perform core workouts for 30 minutes a day, correctly, that should be sufficient. Others with virtually no belly fat may only need five to 10 minutes a day. Then again, always remember to workout correctly as an hour of abdominal exercises will do no good if you perform the movements incorrectly.