A drawing is worthy of feedback if you do not know how to improve it yourself. Give it your best shot, give it a good night of sleep, fix the problems you can, and then ask somebody else for feedback.

Feedback is useful at a number of stages - not just when you're done. Once a piece is done, it's done. Any feedback you receive on it can be used on the next piece, but this one's over. Lots of people seek feedback when they're at a crossroads and aren't sure which direction to take. Plenty of people post WIPs (works in progress) when they aren't sure about something specific. I think when posting unfinished work for feedback, the best things to consider are what specific issue would you like the feedback to address, and simply whether you yourself put a good deal of time into working through your issue on your own first. For the first one, "I don't know where to take this piece" is vague, but certainly still an issue worthy of feedback. If you can offer people something specific to focus on however, that is certainly going to make things easier on the one offering their time. That last one is subjective, but some people have a habit of valuing their time in this context a little too highly, and actively seek feedback early and often as a way to minimize their own time investment. As long as you feel you've made an attempt to think through whatever it is you want help with, then there is nothing wrong with asking for help. Worst case scenario, let's say you post something for feedback and you receive none - that doesn't mean posting your work was somehow wrong, a mistake, or inappropriate. It simply means no one had the time to offer in that instance. No harm done. Ultimately just remember that posting your work for feedback is not a performance. No one is looking for you to meet some standard or to please their sensibilities. They're giving feedback ideally because they want to help people grow. That's all. It's easy to forget that sometimes amidst the eternal pursuit of validation social media engagement.