Depends upon the grain size of the sand and how disturbed it is. If you have no fish in the tank at the moment, turn your filter off and turn off any aeration (bubblers) that you have. Settling sand can take up to a week or more. It is a slow process, but your patience is the key to having a happy and healthy fish tank. If you can endure sand settling, you can endure tank cycling, fish disease, and the other things that people usually get fustrated over and give up the hobby because. Patience Patience Patience!
~~Colesea