How long will marijuana stay in your saliva for a saliva test if you are a heavy user?

Marijuana: Marijuana lasts about twelve hours in saliva but much longer in urine

Up to 3 days: Saliva testing detects relatively recent use and may remain positive for up to 3 days after last use. More commonly used urine tests can stay positive for a month or 2 in heavy users so saliva tests are more accurate to assess recent use. See http://forensicfluids.com/oral-v-urine/