Aspirin stays in the body for at least a few hours, based on studies featured on PubMed.gov regarding the half-life of aspirin. The amount of time varies greatly from individual to individual and is based on a number of factors, including dosage.
The human body metabolizes drugs such as aspirin in four stages: absorption, distribution, metabolism and elimination. The body quickly absorbs and distributes aspirin, which is metabolized into salicylic acid. A Tulane University School of Medicine Web review of pharmacokinetics says aspirin has a half-life of 15 minutes, but the half-life for salicylate is dependent on dosage. Elimination of aspirin is dependent on urine pH, how fast urine flows and other factors, according to a 1985 study in the journal Clinical Pharmacokinetics.