I would recommend you to do not draw skeletons, but the whole body, even at the 30 seconds draws.
Skeletons/component shapes are a fine exercise to work on! :) But of course you're free to try other exercises if you have a different goal in mind. What you've accomplished with self-teaching is very impressive!
Your longer drawings look beautiful to me. I do notice that you have a tendency to really "pet" the lines and go over the same area many times with more and more and more little strokes. I would recommend focusing on trying to capture the gesture with the smallest number of long, fluid lines possible in your next few practices, to start to get a more fluid and confident look in your figures.
You might want to start with a half hour of short poses (1 to 5 minutes) to loose up before tackling the longer poses. I agree with Kim about the long, fluid strokes.
Thank you everyone! I did initially start with full body gestures (as opposed to skeletons) but switched over based on the advice of some tutorials here, I think I'll start incorporating full-body back in and do some of each. :) And I definitely see what you mean about the lines,thank you for pointing that out! I do get a little lost with the longer sketches sometimes, which I think is where the "petting" as you put it gets the most prominent; would you suggest focusing on shading more in that time than on overworking the lines? Or to slow down with the initial proportioning, as it's not limited to 30 sec for longer poses?
If you have spar time, I'd recommend moving on to the next figure. I assume you're drawing figures to get better at drawing figures, so its best to separate adding detail such as shading, until you're confident enough in your figures.
At least, that's what I do.