I wouldn't consider my YouTube venture a career, but I'll bite regardless;
For me it has to be the moment one of my videos surpassed 1000 views. Now I have one that is over 12k (I got a crash vid that is at almost 20k, but I did not do anything but be in the right time at the right place, so I don't really feel a sense of accomplishment there).
I am actually nearing the "big" 1k subscriber mark. It will take a little longer, but I feel I will get there. Even though the numbers are not the main reason I do YouTube, I do understand that increasing numbers mean more people to interact with. And that in itself is epic.