There are no specific answers to your questions. Each person and situation is different and you will be done when you are done. Pay attention to where you are at with each skill you are generating, and generally you will move on when you feel complete.
If the skills are related, you can sometimes work on several concurrently, but just be careful not to overextend yourself and take on too many at one time. This will diffuse your focus and energy and end up diluting your results rather than enhancing them.
All the best!