Random police/detective question

[thatpfunk]

Does anyone know how long the police will investigate a murder that lacks evidence before deeming the case unsolved?

Hypothetical: A drug deal gone wrong, it leaves two or three of the memebers of the exchange dead. Let's assume small/medium time dealers so there is no previous investigation, etc. The police have no "clues". How long until they tell the family members, "sorry, but we are no longer going to be investigating this murder..."

Thanks.
(The reasoning behind this question is completely uninteresting so please don't get excited)