No more than 4 hours. The effects of smoking marijuana usually last no more than 4 hours (though might last a bit longer if eaten). If you're still feeling anxious several hours after using the reason for the anxiety is not directly related to the marijuana. Lots of people who don't use marijuana get anxious too! Try magnesium, tulsi tea, l-theanine, exercise & relaxation techniques. Avoid marijuana. See a doc if needed.