The Fourth Amendment requires that search warrants be "executed," or carried out, without unreasonable delay. (By contrast, arrest warrants are usually valid indefinitely.) The purpose of the rule is to ensure that when a search is made pursuant to a warrant, probable cause exists, both when the warrant was issued and when the search is carried out. If probable cause no longer exists, then the search is considered to be without a warrant, and the evidence seized may not be used by the prosecution unless one of the very narrow exceptions applies.

So the question becomes, what's "unreasonable delay?" A delay is unreasonable when it's no longer reasonable to assume that items that were presumably located in the place described in the warrant  are still there, because the information that placed them in that location is old and has become unreliable. For example, a search for evidence of a single incident of criminal activity should be carried out soon after the warrant has issued, but warrants for evidence of continuing criminal activity (such as ongoing drug operations) generally enjoy more leeway, because the criminal activity is not fleeting.

Police may also delay execution when they are waiting for the evidence to be present at the location named in the warrant. Some warrants specify that the search must be carried out within a certain number of days, and some state statutes give timeframes, such as ten days.