Milk ( casein) paint can be kept refrigerated for one week before use in artwork.
i place 35 g milk powder ( 80% casein) in 125 ml. water. I warm in microwave in 30 second intervals until uniform. I add 35-40 g "air dry" clay and shake for 30 minutes. I then place it in a blender for 4 -5 minutes . I pour the base paint in silicone molds or ice-cube trays and store in the freezer (-15 deg C/ -5 deg F) overnight. I then pop the solids out and keep them in a jiffy bag in the freezer.
When i want to make a paint: I place a unit in the refrigerator overnight to begin to thaw out. The next day, I bring to ambient temperature and stir with dye/ pigment for painting.
YOU can store the units for 1-2 years in the freezer!