Interesting thought. I personally have no real long term goals with regard to playing, therefore I find it difficult to define any sort of ultimate success. I know the idea of having "no goals" might drive some people crazy. And I wouldn't mind being a rock star! But I am not working towards it either. I think I left those days behind a couple of decades ago. :D

I left music for a long time, and never knew what I had given up until I returned to it this past year. I play because it fills up something I didn't even know was empty, so in a sense I have already achieved success.

I guess I measure it more on a day to day basis, mostly after a session. I feel good when it's gone well, when I've slipped in some new move, or something comes more easily, or I just played what I already know with some particularly good inspiration. Or getting the rythmn for a new song, then getting the lead break... Etc, etc. My successes are small when measured against more lofty goals, yet are satisfying to me.