If you really can't keep your child off the iPad, how long should you allow him to play on it? Also, at what age should they be really using the iPad?

my preschooler had gotten addicted to both the Iphone and the Ipad at one point of time and always wanted to watch rhymes and so on. however, we realized it was turning into an addiction and she would only be happy when she had the gadgets on. we made her stop and as of now, she uses gadgets only on the weekends, that too not always, and at the most for an hour in an entire week. she is now 4. i am okay with my 10 year old using it for 2 to 4 hours a week to help her with studies and so on.