Read at first.
There is a group at our russian social networking website, that holds such called "2 hours of sketching" - at certain time the admin of the group gives a theme, and those who want to participate should make several sketches. It's destined to help young artists to sharpen skills. So I decided to participate sometimes. I created a Tumblr so you can look at those sketches of mine if you want.
Here it is ravna-resta.tumblr.com/