How long does thc last in the mouth.I have not smoked since January 20th. Failed a drug test on that day.Getting retested on the 23rd of February but would like to up the time to the 17th of February. The test I was giving was a salty/chalky thing I had to hold up against my cheek for 5 minutes. I did do an at home urine test last night and did not pass (if I even did it correctly). I will be given the same mouth test again.