Think of it as a work in progress, and a lifes' work. It's never really finished. Sure you can gain some skills as you progress, but you never truely master them, at least not fully.
The best you can do is progress as far as your willing to go, if you think you have 'mastered' your skill, then so you have. However, that does not mean there isn't more that you could learn, or ways to improve on that skill or even come up with new things you could do with said skill(or combinations of two skills you put together to come up with something new).