There are a couple of different layers of success for me - even finishing a page feels like an accomplishment, and every little piece of feedback I get makes me happy.
The main reason I'm doing my comic is to get the story down, drawn, and something I can hold in my hands. The telling of it is important to me, the sharing of it too. If my focus ever shifts to number of hits or whatever instead of the story itself, I probably won't enjoy making my comic as much anymore.
That said, I'll feel successful when the first few chapters are out there and done - and really successful when the whole thing is told, because I know I'll have learned so much along the way about making comics.