Here's my two cents.
A lot of people have said that drawing lots will make you better. I don't believe it's that simple. You can draw for the rest of your life and never capture a millionth of the essence of the greatest artists out there. There's always going to be the je ne sais quoi that you either have or don't. But that's out of your control to a certain extent so I wouldn't worry about that too much.
What you should worry about is putting yourself in a bubble. By that I mean, drawing by yourself, for yourself and never showing anyone. That's the worst thing you could ever do for your development. The second worst thing you could do is to think that you'll improve simply by posting your comic on a site like this. 99% of the time, people aren't going to give you constructive criticism. All you'll get, if your work isn't up to scratch, is you'll be ignored. This is why I think LINE is better than here in this regard, there's a voting system, so you can get some real feedback.
What you REALLY need to do is put yourself into a competitive environment and enter as many competitions as you can. Competition is the engine of improvement. There are lots of sites out there that hold official and community-based art competitions. That's where you can hone your craft. For me it was submitting t-shirt designs on Threadless where I really learned my craft. The competition is brutal, and because of that, I've watched dozens of artists on that site go from newbies to pros in just a few short years.
Back to comics, the thing about them is there are a million different little things you need to master to get the whole thing right, so whatever you do, don't spend an inordinate amount of time on story/characters/plot while you're still learning to draw anatomy, make panels, do backgrounds etc.
So yeah, it's a fine line, but whatever you do, make sure you draw as much as you can, and always put yourself in a position where you're competing with superior artists. That's how you'll get better. As to when you should start doing all this, as soon as you think you'll be able to put all the pieces necessary for a comic together on a regular basis. That's no small task and may or may not be the best environment for a beginner artist, that depends on your own personal level of tenacity.