Grout takes 24 hours to fully cure and dry. When grouting tile, it's best to let the grout cure for 30 minutes before wiping each tile with a slightly damp sponge.
According to This Old House, grout should be mixed according to packaging directions and allowed to set for 10 minutes. This time allows a chemical reaction called slaking to occur. Slaking is when the pigment, cement and sand combine to create a consistent color and thickness. After 10 minutes, grout should be stirred again and then worked into the tile joints using a rubber grout float held at a 45-degree angle.