I leave the crayons, markers, and paper out all the time. They mosey to the table to make pictures, write letters, or create signs. Sometimes it is coloring books, other times just plain paper. Although there are no formal invitations, it usually becomes a group endeavor, especially as I prepare dinner.
About twice a week we really go at it. Our creative juices are squeezed out with more in depth projects - and of course, the messier the better. For each half hour of my preparation the projects will last about 10 minutes...par for the course at this age and I don't let it bother me. The 45 minutes of cleanup, however, is a little frustrating. I know I have hit the jackpot of crafts when they are still at the table after an hour and asking for more.
I would like to maintain the twice a week, and do my best to up it to 3. Even after they start school full time. On the off days I would love to put down whatever Im doing and join in on the coloring, or the building, or the creating. It is awesome to see their little minds work. And I feel like I get to know them a bit better when we create together.