After about an hour the concrete should be firm enough to imprint circles about 1/4-inch deep with the tops of cans. After about 6 to 12 hours (depending on the weather), place a board or tray on top of the mold, invert each stone onto the board, and carefully remove the saucer. Cover with plastic for 24 hours, then gently round the edges of the stone with a file. Cover the tones again in plastic to keep moist for at least five days to cure. The longer the drying time, the stronger the concrete, so always keep concrete out of direct sun while it's curing.