Perhaps my favorite image in the wonderful book "Daily Painting" by Carol Marine (link) is that of the ten minute apple studies by Carol Marine. She describes her exercise in the text . She also describes it in one of the art challenges on the Daily Paintworks site (link),
This Ten Minute Challenge was our class exercise this week.
The classes were asked to bring a simple object to paint. Once seated, they were told to ready their paints and draw approximately 3 inch squares onto a piece of practice canvas. The timer was set for 10 minutes. They were instructed to paint their object using a minimum of brush strokes, to not over blend their colors and to paint shapes, colors and values. (not named objects). They were told to expect that their first attempt would be bad.
At the end of ten minutes they had to stop.
Then a second 10 minute painting of the same object was done. The object could be moved for the second study.
The class was surprised at how long 10 minutes actually was, how lovely and fresh their little ten minute paintings actually were and how much they enjoyed doing this exercise.