spit test

ok i have a good paying job that very rarely will give a saliva swab test, they started doing it last october and i have been clean since march. i decided to grow anyway this year and my c99 and bubba kush is looking to damn nice and its gonna be hard to not smoke it. i have heard that the detection for thc is only 12 to 48 hours in a spit test, my question is can anyone tell me this is true, and also can anyone tell me anything to do or take to pass one if there is a chance i can fail it, please help a guy that will have a alot of homegrown than may not be able to do anything with it.