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 till beginning of the next day, to receive new Quests.