If the original concentration of a drug in a patient's bloodstream is c (milligrams per liter), t hours later the concentration will be C(t) = cekt, where k is the absorption constant. If the original concentration of the asthma medication theophylline is c = 10 and the absorption constant is k = 0.23, when should the drug be readministered so that the concentration does not fall below the minimum effective concentration of 5? (Round your answer to two decimal places.)