When you ask the question “How long does marijuana stay in your system?” you need to take into account the fact that there is no simple answer to this question. THC (Tetrahydrocannabinol) can stay in a person's body for as long as 3 to 90 days after smoking or being ingested orally. There are numerous determining factors for how long drug toxins stay in a persons body which vary from person to person, such as the analytical method used, your health, your body weight, metabolism, fluid intake, the type of drug toxin, and the degree of exposure to the drug toxin.