Grout essentially dries out in 30 minutes once mixed. Once completing the tiling, allow 24 hours for the grout to cure completely before using water near the tile, such as taking a shower.
People who are new to grouting should work with small batches of grout because of how quickly it dries. Otherwise, the grout dries in the bucket or other places on the wall. Also, the excess grout should be wiped off quickly so that it does not dry on top of the tile. One should only wipe the surface of the tile, avoiding the grout set into lines between the tiles.