I'm using left-over paint from last weekend's adventure at Unforgettable Art. This is painted on a 5 x 7 gallery wrapped canvas.
There's a lot more paint on the palette. I should use it soon. The paints draws up moisture from the sponge below the special palette paper and the paint becomes a bit watery.
The other reason to use it soon is that the paint can go bad. A neglected palette can grow mold.
You don't have to buy a special palette to keep your paint moist. A sealed container with a wet sponge in it will keep the paint moist. The advantage of working on the moist palette is that you can have several colors out in small quantities. They stay workable instead of drying.
Well, I better get back to painting!