How many days a week should I do abs to get an eight pack?

Abdominals are just like any other muscle group (chest, back, legs etc). You need to work them as you would any other muscle.  If you want them to grow, then you must dedicate some time to them.  I would suggest doing at least 2-4 different exercises, 3 sets of 12-15 reps, 2-3 times a week. Some good ab exercises include: ball crunches, reverse ball crunches. oblique rotations, jack knives, hanging leg raise (1 and both legs) and planks. Watching your diet will help as well. Avoid processed foods, sugar added drinks and alcohol. Eatting lean meats and vegtables will also help trim away that belly fat that covers them up.

Most people don't realize that they have defined ab muscles underneath all that fat.  When people are wanting a six or eight pack the number one thing that it comes down to is diet.  In the end it really won't matter how hard you work your abs if your diet is not intact.  Now your abdominal muscles are like any other muscle group and you should only work them every other day because they will need time to heal and repair until your next workout. 

You probably don’t need to exercise your abdominals as much as you’d think.  Just like any other muscle group, you want to have at least one day of rest in between your abdominal workouts. The muscles need time to recover and then grow. You will be most successful if you work your abdominals 2-3 times per week and perform exercises from all planes of motion. I like to do 2-4 different exercises on the days I work my abdominals. Some examples of exercises you can pair are planks (include side planks as well to work oblique’s), ball crunches, crunches with rotation and even other core exercises like the floor bridge. With that said, diet is one of the most important factors in getting an eight pack (or six pack, etc). You must reduce body fat low enough to see the toned muscles underneath. Make sure you are creating a calorie deficit to reduce the body fat covering those toned muscles and you will achieve the look you desire.

There is no such thing as "spot reduction" and in order to get an eight pack depends on a delicate balance between exercise, diet and genetics. Having the proper exercise routine will help to build your abdominal muscles but you will need to eat the correct diet and have the proper weight loss (for example avoiding excess alcohol or foods that lead to gastrointestinal bloating or weight gain) and also there is a genetic component to the way (even when the abs are exposed) they will "look". 

Doing ab exercises is only one part of the equation.  If you have too much fat around the mid section your abdominals will never show no matter how many ab exercises you do.  Be sure that you are eating right and doing some cardio to also burn and lose fat around the mid-section.  Now when it comes to specific exercises I would recommend not to do so many crunches.  Most people over do this exercise and forget that the abdominals, like all other muscles, work in all three planes of motion (saggital, frontal, and transverse) and not only works concentrically (like in a crunch) but eccentrically as well.  

Just like your other muscles, the abdominals require rest after workouts. The more important factor to focus on to let your abs show is your diet. Are you eating properly? The abdominals are used with every exercise you perform. The core forms the foundation for all movement, so, if you’re working out, you should be engaging your core with the various exercises you perform. If you are focusing strictly on the abdominals, perform 2-3 sets of 8-12 repetitions at a moderate to fast intensity. Do this 3 to 6 times per week, allowing at least one day of rest.

Diet is 50% of it. A lot of people have strong defined abs, but they are carrying so much belly fat that you can't see them. I love doing abs and ideally you want to do them every other day. There are 15 million different kinds of ab exercises. Variety is key, learn as many different kinds as you can, so your muscles continue to grow. Take classes-so you pick up the energy of the class and do more than you could possibly dream of.

You must absolutely cut body fat to see the abdominals. The "eight pack" that you refer to is there. It is there for everyone. We need only to reduce body fat enough that it becomes visible. The question is not which abdominal exercises do you need to do and how often. This of course is relevant to the amount of thickness within the muscle itself but if that muscle is covered by a dense layer of fat it makes no difference whatsoever. This question requires a strict eating regimen to reduce body fat while at the same time a smart abdominal program to maximize the growth of the abdominal muscle bellies (hypertrophy).