Mouth swabs are not the common way of testing. Are you sure that is what they are going to do? Depends on if you have taken a long acting methadone or short. The short can last for about 5 days and long acting I am not sure about. It also depends on how often you have taken them. If it is just a 1 X thing for short acting it might be out in a day to 3 days. Check out drugbuyers.com maybe more help for you.
