This is a tricky question to answer, because there is no set standards. Moreover, all people are different and you also need to consider the age of the person in view. Here are some general points.
First, there are not enough tests to determine the long-term problems associated with many electronic devices. Part of the reason for this is that much of technology is relatively new, like the cellular phone.
Second, it is a general rule of thumb that the less time you send on these devices, the better it is. For example, it is not good for a person's posture to be on a computer for a long time. If a person has to be on a computer for a long time, he or she should also stretch.
As for children, most parents I know do not let children on a electronic device for more than 1 hour a day and they really do not like children to use cellular phones.