I think getting your art out there for critic as early as possible is goodBut find places that actually gives critic tho, like art groups, art forums. Places like facebook and deviantart is great, but in the end its more about showing art than actually getting critic and learning. You will be amazed how much other peoples opinions can help you improve. When to start promoting your art is a hard question. First of you need to be comfortable with the level it is at, and at the very least it need to look finished. Dont remember who said it, but a talented artist said that when you can put your art together with other well know artist or professionals and it dont stick out like a soar thumb, you might reached a level that is professional.hahahah, no dude, would not really call myself talented. I just did a lot of crappy work before i did the less crappy stuff i´m making today. It is really that simple. You want to be good at character? Cool, then do a lot of characters, compare yours to other artist, see why other artist are better than you, study it and repeat. Good luck on your art dude and just keep on pushing forward