Look up the "half life" of the drug you are taking. If this is, for example, 24 hours, it will take one day to clear half of it from your system. It will then take another 24 hours to clear half of the remainder (leaving you with a quarter dose in your body). A further 24 hours will, once again, remove half of the remaining, leaving an eighth. A doctor I discussed this with said that the rule of thumb was three half lives for a drug to become ineffective.
Sally