We have a class at Art Center called Digital Landscape, with Nick Pugh.
It's a really great class, we go on location and paint places with a different focus (sometimes architecture, sometimes water at Long beach, etc.)
I was on the fence whether or not I should post these, as they aren't pretty and I still have a long way to go to get better, but hopefully some of you without the curriculum will try this yourselves!
Nick has us paint in the realm of 20 minutes per. Occasionally they are under 10, I think those speak for themselves :P Then once in a while there's a longer one, about 40 minutes is the absolute max.