The elimination half-life of seroquel is 6 hours which means it isn't completely out of your system for 30 hours. There is some effect during the day, just a smaller amount of the dosage so first six hours 300mg, beginning of next six 150mg, beginning of the 13th hour - 75mg; and so forth. At least that is my understanding of half-life of a medication. It takes five times the half-life for any medication to leave your system completely. That is why some drugs don't have to be tapered, because their half-life is long.