It can take a few weeks all the way up to a few months. It depends on what analysis they are doing on it and what they are actually testing. Moreover, a secondary analysis can occur. Once the labs have been obtained this does not mean that a warrant will be issued immediately, that too could take some time. This is all dependent on the lab tech and the prosecutor.
Joshua's answers this question about how long. To answer it another way, delays are usually advantageous in criminal cases. It would be my policy to not rattle any cages by calling the police or drawing any attention to the file. Without being specific: Sometimes, long delays can make certain evidence less compelling or stale. In addition, while there is probably no issue with a statute of limitations, an attorney will may use the delay to fortify your position and may have a due process issue if the delay is significant and hinders the defense. Good Luck.