I think it depends if they are pushing away other aspects of their life because they are on the internet all of the time. Friends, families, social engagements...
Addiction isn't really defined by the amount of time you spend doing something but rather how you feel about the behavior and how it affects your life. You can spend just an hour or two on the Internet a day, but if you feel like you have to do it and you do it even when you're supposed to be doing other things, you might be addicted. It does seem logical that the more time you spend on the Internet the more likely you are to be an addict--for instance, if you're spending 8+ hours a day surfing the web it's probably more likely to interfere with your life than if you spend half that time. Still, being addicted is about whether you do it compulsively and despite negative consequences, not a certain number of hours.
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