15 Seconds

That’s all the police have to wait for you to answer your door before they break it down to execute a warrant, at least when they’re searching for drugs:

His case gave the court the opportunity to clarify how long police must wait before breaking into a home to serve a warrant. The court ruled 9-0 that a 20 second delay was ample, because any longer would give drug suspects time to flush evidence down the toilet.
. . .
[Writing for the court, Justice David H. Souter said] while “this call is a close one, we think that after 15 or 20 seconds without a response, police could fairly suspect that cocaine would be gone if they were reticent any longer.”

Given the facts of the case (the full decision is available here), this ruling seems reasonable. However the opinion should have used stronger language to distinguish the rule here from non-exigent situations. For instance, Justice Souter writes:

Police seeking a stolen piano may be able to spend more time to make sure they really need a battering ram.

May be able to spend more time? How about must spend more time?
At any rate, if the police come to your home searching for drugs, you better hope you’re not stuck in the shower.