Drug testing question

[420feetuphigh]

Ok, Last night i smoked a joint, containing really good bud, and rolling tobbaco

It was my first time smoking weed in over a month, so meaning untill last night i have been 100 percent drug free for a month and a week.

I am subject to random, home drug tests, meaning the ones from the drug store or on line. I could get one today tomorrow or the day after.........or the day after, im guessing i have at least 3-4 days until the piss test, but it could be before that,

The question is, How long dose it take to get THC out of the system with only using it once in the course of a month,

I used to be a daily pothead, but now becouse of the issues i am in, i can only do it monthly,

So how long will it take to get rid of THC based on the situation a shared,
Do i wait, or do i drink a shit load of water every day assuming i have the test that day, i cant fail the next piss test comming up, or ill be fucked
any ideas, btw the piss test are random, and supervised