i lost my job over thc in a random last september. i'd gone 4 full weeks without having smoked. technically, the tests go back 45 days. my best advice is work for yourself which is what i do now, or carry fake piss. because getting that phone call is never fun.
Most urinalyses post employment are monitored, so fake piss would get you fired for sure completely apart from the fact that fake piss doesn't have hormones, urea, nitrate, creatinine or any other wastes commonly found in urine.
Tests don't go back 45 days. Tests simply measure the amount of cannabinoids that are present in a sample. The parameters can however be adjusted based on the demands of the employer. 1 ng/ml of thc metabolites is detectable for months after ceasing usage with constant use. Everyone's metabolism is different. You cannot and should not expect to be completely clear (under 50 ng/ml which is the common cutoff) of thc metabolites within the same amount of time as someone with a metabolism that is faster than yours. Also, where you work and the nature of your job is critical in determining how specific your employer will ask the testing agency to be.
The agency collecting the specimen (aka your piss or hair) can be told to test down to 1 ng/ml of thc metabolites. If you're getting tested to 1 ng/ml it is likely for one of two reason 1) questions of character or 2) liability purposes/insurance. If it is your hair that is being tested, then three months of sobriety is the absolute minimum.
If you're flipping burgers, your boss very likely does not care what you do on your free time. If you handle money, medication, people or expensive machinery then you should expect your boss to care. Do not be fooled by the uneducated plebes who will convince you that "you're fine bruh, just don't show up to work stoned."
Even if you are below 50 ng/ml which is the usual cutoff, you can still test positive depending on the nature of the urinalysis. If you have smoked within the past 30 days and you are not very active, then you might as well quit because you will likely fail the urinalysis. Otherwise the option is to get fired since most jobs consider refusal of a urinalysis as basis for termination. Long and short,
don't be stupid. Don't share information about your usage with people you don't trust. I don't share any information with people unless I know them well and I know they're down with the 420.
Don't fall into the stereotypical stoner archetypes, because that shit will get you tested randomly for "question of character/ culture of drug abuse." For everyone else who exists within mainstream society, working for yourself is by and large not an option.