You asked, "is opening of the hip externally supposed to be a long process? How long? I guess it depends on lot of things doesn't it?"
The answer is — it depends!
It depends upon what is stopping you. If you are feeling compression in the outside of the hips when you try to bring knees down while in Butterfly, sitting cross-legged or in Shoelace, this is most likely the neck of the femur impinging upon the rim of the hip socket (acetabulum.) If compression is stopping, you are never going to get your knees down...the process will be infinitely long (or at least until your next lifetime
). However, if what you are feeling is tension in the inner groins, that is likely stiffness in your adductor muscles or other fascia there. Overtime, and it may take months or years, that can be worked on and reduced.
Everyone eventually hits bony compression and that is the end of the game. But until then, you can continue to work on reducing tension in the muscles and fascia which may slowly improve your range of motion. But, pay attention. When you start to feel that compression arising at the back of the hips, it is tome to stop trying to go further. You will have reached your ultimate limit.