Last time you should have drawn the below photo in pencil with no shading.
For today's drawing I want you to pull out a pen and begin shading the faces. It doesn't matter which one you start with, but I want you get as much done as you can in the given 15 minutes. I will be looking for quantity in addition to quality.
Don't worry about it looking "perfect" since that isn't enough time to finish something, but you have the skills to make it look good, I know you do.
Why this, well, I want you to get practice knowing that you can re-visit ideas/drawings in your sketchbook and add to them. A lot of little increments added up makes for a big final product at some point.
Set your 15 timer and get to it!