Internet Addiction is not a function of the AMOUNT of time spent online. For example, many people today are using the Internet all day for work, research and school. But they are not addicted because they are not using the Internet compulsively or struggling with problems due to their Internet use. The Internet is a tool, like anything else someone may use for their job or studies. Those of us who still write with pencils -- are not addicted to using pencils. When we are done using them, we can let them go and not continue obsessing about them when we are not using them

In general, addiction is about a compulsive need that interferes with a person's ability to function normally. Quantity is not always the most important parameter to check. There are people who can drink every day and not become alcoholics and people who drink on weekends only but do have a problem with alcohol.
I believe that when someone starts asking themselves if they are addicted -- it is usually indicative that a problem exists because it indicates that they feel they are abusing something. In the case of the Internet, parents need to be involved with their children's internet use from the start and not expect a child to set their own limits. The limits need to be not only about the time spent online but also about what activities and content can be accessed. Certain online activities can be very addictive. I write a lot about the topic here :
http://www.internet-addiction-guide.com