For me, my main measure of success is what people say to me. If people respond to my comics and like my characters, then I feel like I'm doing what I wanted to do!!
Monetarily, I don't have a specific goal (like "I want to be supporting myself with my art in 5 years" or anything like that) so there's no real measure of how far along I "should" be at any given time -- it's more that as long as the things I do are worth the money I'm putting into them, and as long as I'm slowly improving at marketing my work in different ways, I feel that I'm doing well!
I guess I don't have a huge sense of where I "should" be. My first comic wasn't one that I started with the intention of gaining a huge audience -- I did it for fun and for my friends at first, so the following I did get later was a cool bonus! For my comic now, I don't have a concrete goal... I just want it to be as good as I can make it, and I want people to read it and like it. As I get more of an audience on social media, I want to share my webcomic with them, too. And every time a single person says "oh, I just found your comic, I really like it!" then I feel successful, tbh.
Whether or when I get noticed by the right people and grow up to have an audience of thousands feels like it's more in the hands of God than me, so I don't spend a whole lot of time wondering how to get there. xD As long as I'm slowly getting better and making something I'm proud of, it doesn't really matter!