You can make fairly good progress by working a stretch only 1-2 times a week. I have not read Pavel's works on flexibility yet, but I come from a calisthenics/handbalancing world so we need a lot of active split flexibility.
The basic rules of flexibility are :
-stretch one side of the joint (e.g. agnoists)
-strengthen the other side of the joint (e.g. antagonists)
This is because once you strecth into a new ROM, no matter how small an increase, you need to teach your body that it can safely enter that end range. It won't do it unless you have the strength to do so.
So for a side split, you would stretch your adductors and then strengthen your medial glutes
in their end range. Back when I was in my teens and practiced kung fu, the combination of stretching splits and then doing "kick-and-holds" (holding a kick as high asyou can with good from) served this function.
Same for say, shoulder flexion: you need to stretch the pecs, lats, teres major, etc, and strengthen the shoulder flexors and upward/external rotators, such as the upper/lower traps, external rotator cuff muscles, and so on.
In any mobility training, it is of utmost importance that you do it with correct form. Never force ROM by compensating. You will only stagnate your progress, or worse, injure yourself.
As to a GTG approach, you can likely make good flexibility gains by doing moderate stretching most every day, as
@Steve Freides suggested. I would add to the routine by doing something to strengthen your ROM in the direction you are trying to go, if that makes sense.
Some people (myself included) prefer to do longer, focused stretching sessions a few times a week as opposed to every day.
*
If you are doing deep stretches and strengthening your end range, you should see gains.
*I do light stretching almost every day, but do focused flexibility training 2-3 days/week.
Hope some of that is helpful.