They say it is completely digested and passed through your system in 8 hours from your last drink... But it could take longer if you have drank a lot over a few hours. I'd say give it at least 9-10 hours from your last drink to be safe, if your job is on the line or something.
I take a urine test every month,drink 10 t 12 beers the day before the test never failed.About 12 hours sounds about right.1 hr per beer. Just drink a quart of water an hour and a half before the test.