The plasma half-life of aspirin is around 15-20 minutes. However, a metabolite of aspirin called salicylate undergoes Michaelis-Menten kinetics, wherein at low doses, it goes through first order elimination and the half-life stays constant at 2-3 hours, and in higher doses, at 15-30 hours. Because of this, it takes 5-7 days before a steady-state concentration is reached, and about 7-10 days before it can be completely eliminated out of our system. This is the reason why doctors advise patients to stop taking aspirin at least 7 days prior to any surgical procedure. Nevertheless, there are drug tests which can actually detect trace amounts of aspirin 30 days after use.