The question "how long does THC stay in urine" is a complex one to answer. Not all urine tests are the same, not all people are the same, and (obviously) not all marijuana is the same. Yet, as allusive as THC might be to understand, we can still make some general predictions as to how long it will stay within one's system.
For starters, tetrahydrocannabinol is a psychoactive component of cannabis. THC is the main psychoactive substance of marijuana, producing a variety of effects that affect most parts of the body. Those who consume THC might notice that their eyes become reddened, their body becomes relaxed, and increased heart rate. Because it can linger in various components of one's system for a while, it's tough to say just how long THC stays in urine.
How long THC stays in urine depends on you. Bodyweight and previous exposure to marijuana are some of the main factors figuring into how long tetrahydrocannabinol can stay within your system. Larger individuals generally have more fat cells to store THC metabolites in than smaller ones.
How long THC stays in urine depends on the cannabis smoked. Not every joint is the same, as many marijuana aficionados will attest to. Leafs with significantly more crystals possess more THC molecules, and thus more tetrahydrocannabinol byproducts. The byproducts of THC are what stays in your system after the actual substance has worn off, making one vulnerable to a drug test even if they aren't under the influence anymore.
In the end, past usage of marijuana is what sets off the alarm. Sometimes, albeit very rarely, first-time smokers can get away by passing a drug test; this is due to the fact that they only have a few THC byproducts ("metabolites") in their body. Thus, it's quite possible for a first-time or light smoker to pass a test within a month or two. Heavy marijuana smokers, however, have a long history of THC collecting within their body, and it's nearly impossible for these individuals to pass a test without many months of smoking cessation.
Marijuana, although legal in many districts, should be abstained from by individuals from places where it is banned. Regardless of whether or not one enjoys THC and its various effects, tetrahydrocannabinol metabolites stay within one's system for months. Never consume cannabis prior to an important event, job interview, or doctor's appointment.