The good thing about oral fluid tests is short detection window. I have seen it reported to be as short as 12 hours, as long as 96 hours, depending on the source and brand of test. Let's split the difference and say 48 hours clean would be a good starting point. The longer the better. Before going in for the test, scrub your mouth, teeth, gums, inside cheeks, and tongue with your toothbrush. Gargle and floss too.