Your drawings are very good considering you are a beginner, so don't worry too much, eventually you will know how to handle better your time, just keep practising.
You may try to clean a bit your lines. For example, the 5 minutes drawing it's a bit hard to understand. Instead of define the figure by adding more pressure with your pencil, you can use an H (4h, 2h or h) for the sketch and a soft pencil (hb, b, 2b, etc) to finnish your work. Or simply try to avoid doing a lot of lines. In this way you will be able to see better what is wrong.
I like how the last draw looks (30 min), but I can't see many photos and the 30 seconds one is wrong.
Do you do gesture drawings? Check the site's articles about it if you didn't do so, they're very helpful.
So needless to say, keep working. :)