Ambien (generic name: zolpidem) is a medication with a hypnotic effect. Although it does not belong to the class of benzodiazepines, it shares certain of their pharmalocological properties due to similar chemical structure. The effect lasts about 4-5 hours after taking it; where as the elimination half-time (the time needed for half of the quantity to be removed from the organism) is reported to be 2.2 hours.
The exact time needed for the employment urine test not to be able to detect the zolpidem in the urine is not known. However, given the facts above, especially the short elimination half-time and the time of effects, there is a possibility that the urine test would not be able to detect the zolpidem if there is a time gap of more than 24 hours between the pill and the test.