Bacteria can start growing in as little as 2 hours, but you probably won't see it until at least 24 hours. Some bacteria can take longer to see, but you should definitely have something within a few days. The bacteria need a proper home to grow :warm and moist. Make sure the petrie dish is placed upside down in a warm place: this helps keep the agar moist without drowning the bacteria. The warmth will encourage the growth, but be careful not to put the petrie dishes into direct sunlight since sunlight can kill most bacteria (UV lights are often used in hospital hallways to help attack bacteria and stop their growth).