<image> In a pharmaceutical research institute's experiment on the efficacy of a new drug, it was discovered that when adults take the drug according to the prescribed dosage, the drug concentration y (micrograms) per milliliter of blood changes with time x (hours) as shown in the figure. If the drug concentration per milliliter of blood reaches or exceeds 3 micrograms, the treatment is considered effective. Then, the duration of effectiveness is ______ hours.