In a unanimous 9-0 decision, The US Supreme Court ruled that police can knock a door down after 20 seconds of waiting if there is reasonable suspicion drugs are inside. The Court says allowing too much time can allow suspects to get rid of the drugs.
LaShawn Banks filed the case in 1998 after police broke into his house while he was showering. Police discovered 3 guns and 11 ounces of crack cocaine. Banks got 11 years in prison but got out because a lower court disagreed with police procedures.
The lower court stated the 4th Amendment requires police to give suspects notice before they conduct a search. The Supreme Court says Banks did get adequate notice. The Court did refuse to define a notice time for all knockdowns. It is case by case.
http://www.cnn.com/2003/LAW/12/02/scotus.police.searches.ap/index.html