If we are talking about need here, then the answer is zero.
You don't NEED to train abs to get a six pack. Ever. Never ever.
You NEED to get your body fat down to a level that shows your abs.
Your rectus abdominis, obliques and other "Ab" muscles are all there. Somewhere.
You just need to get them out...
So why do I NEED to train my abs?
Again, you don't need to, if you are doing the big lifts and are very physically active you will already be working your abs a lot. Squat training with heavy loads for example is a fantastic way of training the entire core.
So why train my abs?
Research has shown that training a particular muscle results in increased levels of blood flow and lipolysis in the area (the breakdown of fat cells into energy you can use), however the difference it makes does not warrant it for reasoning to train abs frequently.
Training muscles burn calories but these calories don’t directly come from the fat covering them.
Abs like other muscles can be trained, you can grow them, strengthen them and improve their endurance.
These three things have benefits for supporting the rest of your training and you can increase the size of your abs to improve visibility.
But only if your body fat levels are low enough to see them....
Strengthening your core will improve your ability to perform the “big lifts” with a larger weight and increasing the strength endurance of the abs will help you lift for an increased number of reps on certain movements.
Common problems I see.
I train abs 5 days a week and can't see them?
Are you eating a calorie deficit to drop body fat? Could your time be better spent on more productive gym movements. Or could you use this time for something else entirely? Like getting your nutrition right...
My back hurts from all this ab training
A lot of traditional ab training isn't great for your posture, also if overdone it can aggregate or cause injuries. Make sure you train your body evenly and keep your hips and back supple with flexibility work.
I don’t think I am training my abs enough
Enough for what? To see them? Worry less about your ab training and more about your energy balance, nutrition and overall training.
What is ideal for ab training?
What is great for one person may be terrible for someone else. We don’t all share the same goals, ability or capacity for the same movements.
If your goal is improving the strength of your abs to support your training then working in the strength rep ranges with more challenging lifts such as barbell roll outs will be of great benefit
If you goal is definition then focus first on getting your calorie and overall training right. You can then use ab training to help supplement your other lifts and to encourage muscle maintenance/development in the abs during your time in your fat loss calorie deficit.