An individual cannot gain abs in one day without the assistance of surgery. Gaining visible abs is a process that could range anywhere from two weeks to two years or longer, depending on a person's current physique.
The most important factor in getting abs is dieting. This includes not overeating, eating healthy fats and carbohydrates, and getting plenty of protein. The second most important factor is the exercise that works with the diet. There is no need to do thousands of crunches when compound exercises work multiple muscles and burn more fat. If an individual maintains a proper diet and a consistent workout plan, abs develop.