Hi, I think your work is nice, some faces and expressions are not good but some others are really well done like the 7th, the 8th and the last one. You should try to work with shapes to improve (shapes created by lines of the face and its volume). D'ont hesitate do draw the shadows (and light).
before drawing lines, when you are starting to draw the face, make an oval of the shape of the head and place some important elements like the nose and eyes but just with a few strokes lines.
You can also practic by drawing the same figure many times, until you're bored, but you'll see that you'll really improve (it worked for me !).
I think your face and expression drawings are completely but totally expressive but solid, in terms of them being done for 60 seconds each, but I feel that these faces and expressions all seem too rigid to me yet. How would you like to please go for 16 minutes of 30 second faces and expressions sketches, all just to completely and totally loosen up your forces and forms of those faces?
The reason why is because, your accuracy and speed will constantly improve with much more constant practice, even if you're past 10 hours of drawing practice or more, but to make your facial forms and forces less rigider, but all the most spontaneous, fluider, but more human yet.
For most info, look into the Andrew Loomis book on Drawing Faces and Hands, for more practice on drawing faces and more, so hope these have helped you out the most.