Darker granites will only need sealing a little often. Every year or two should suffice. They are denser than light ones. Besides, any stain is usually nearly invisible because of the stone’s darkness.   Light-colored granites, on the other hand, must be sealed yearly.   For marble, onyx, sandstone, and travertine, once or twice a year are advisable.   As a general rule, if you notice that the stone darkens when it comes in contact with water, that’s the time you should reseal.   If you notice that the stone darkens when it comes in contact with water, that’s the time you should reseal.