It can only be watered once every 24 hours, but that doesn't mean it grows every time you do. You will get this text after successful growing: Your plant has grown to the next stage!, otherwise, you will see only: You watered your plant., and if you try to water it before 24 hours have passed since last watering: Your plant doesn't need water. After it's fully blooming, your plant doesn't need to be watered everyday, however if you don't water it for too long, it will turn into a withered plant, and only fast watering saves it from disappearing: You finally remembered to water your plant and it recovered. The plant will be restored to 3rd stage. However, if you haven't water the withered plant for a few days, it will turn into an Empty Flower Pot. You can water it by using a Watering Can.
Note: The plant's timer is not counting while you keep it in depot box when it is closed, or in your backpack when you're logged out so it will never grow this way.
Mirade confirmed this: Every plant can only be watered once every 24 hours. After 24 hours it takes 5 days until the plant turns into a withered one (if you don't water it) and then it still takes 2 days before it will be ruined completely.