You may receive up to 3 instances of Daily Quests per day, with a delay no less then 6 hours between each refresh. In other words, if you complete all your Daily Quests of a certain type, you will receive your next portion of Daily Quests in 6 hours.
However, if you have already completed 3 instances of Daily Quests during the day, you will have to wait until the beginning of the next day to receive new Quests.