If you are a beginner, try this abs workout that lasts between 15 and 20 minutes.

There are 7 exercises, with 3 sets each. The rest between sets is 1 minute. 

The rest between exercises is 90 secs.

Advanced abs workout consists of many supersets and shorter rest between each round of a superset.

If this workout is too easy for you, try the following (advanced variation):

1) All 7 exercises represent a superset (meaning: exercise 1, no rest, exercise 2, no rest, exercise 3....). There is no rest between exercises;

2) Do 3 rounds of this superset;

3) the rest after each round of this superset is between 3 and 4 minutes;

4) make sure to follow the rep/time range of each exercise (for example, exercise 1: repeat for 30-45 secs, exercise 2: 40 reps (the time here is not important, but rep range is)).