Here's some great info by the master FBR:

"Just for a little more clarity, most oral fluid tests have a detection window of about 24-36 hours but I have seen some brands that can go back 48 hours and some even 96 hours. So stop 4 days before the test and if you can't then make sure you brush your teeth and gums and tongue very well before you take the test. You should also position the swab so that it is barely touching the soft surfaces in the mouth."

Hope this helps and good luck!