The amount of time that weed stays in your system depends on the user and how frequently they smoke. There are other factors involved, too, such as how it is being tested.
Someone who has smoked for the first time can test positive for THC for up to 8 days in urine.
Someone who smokes frequently can test positive for THC for up to 15 days in urine.
Someone who smokes heavily can test positive for THC for up to 30 days in urine.
Some heavy marijuana smokers have been tested positive for up to 45-90 days in urine after stopping smoking cannabis.