I have gone a long time with my layout partially completed. It is mostly in details, more trees, etc. that it needs work. In the interim, I find a good day is when I can manage to generate a decent image, sort of a hobby within the hobby for me. Now that I have begun to develop, I can see the deficiencies in the layout more, so I need to get busy and do some sprucing up this winter. But, for me, a good day is when I can produce a nice tree, or use them later in a scene and call it a good one.