Good question. It would depend on what the computer time is used for. For example, is the teen engaging in school work, research, any sort of cognitive stimulation or is the time spent in chat rooms, FB, online games, etc.
It is best to have a reasonable expectation based on what the teen is getting out of the computer time.
A reasonable way to communicate this could be- pointing out that the computer is a tool and not something to be used to fill in time or as a defense mechanism. Spending extensive time on the computer if it is not related to work/school projects should be within the time frame of a couple of hours a day. There are other activities that the teen can engage in that involve social face to face interaction, physical work (exercise) and helping around the house.
Something else to point out would be that the computer can't replace certain things and that escapism is not an effective way of dealing with things. For instance, typing an email is not the same as a phone conversation or the actual face to face interaction. The computer time can be seen as means to achieve something without placing 100% reliance on it.
Physical inactivity (sitting down) throughout the day using the computer, can lead to obesity, weakness, poor social skills/possible depression as the individual is not engaged in social/pro active activities.
You may point out that the computer would always be available but time spent with friends and doing other things can change over time. People may relocate, individuals can become busier w/ their job (eventually it will happen) Hence, it is better to take advantage of doing other things and engaging w/ others (even volunteering, a pro social activity) rather than being consumed by the computer.
As far as enforcing this, try to do it through encouragement rather than consequences; ask for help at home or going to the store, encourage going outside or participating in clubs/sports, having friends over, etc.
You could also share some studies in regard to what "negative" outcomes /health related issues there may be due to extensive use of computer time.
Dr. Rossi hello and thank you for your very informative answer. I just wanted to add a few more things that can help clarify the situation: Luca has very high grades in school and does sports every day in school, he is doing cross country races. He does chores, I was not able to invent new ones though, maybe I should focus more on this. He says quite adamantly that if he completes all his tasks he should be able to use his time as he wants, in this case playing strategy computer games, which are usually about administering some tight resources and going to war all the time. Maybe if I found some games that can develop him more, but still I do not want him to play more than two hours during weekdays, the weekends are still under discussion. He was very upset when I started talking about the two hours limit. He says he does not have anything to do, I told him that he can read or watch Discovery etc. It is true that the interacts a lot with his friends during the games, and we live at the country side where he did not seem to manage to have more of in person relationship with children, their friendships are being experienced during sports and video games. I look forward to your advice.
Thank you for the additional info Larisa.
Depending on his age, he could look into activities at the YMCA or some volunteer ones in your area. It is easy to get engrossed working or playing on the computer. If his interactions w/ peers is limited because of your location, he's most likely compensating by playing the games. His time on the computer must provide some kind of a reward to him in order for him to engage in it to the extent that he does.
The suggestion of using educational sites/games is a good one. If he is to do so, perhaps you can ask him to share what he had learned while on the computer.
There are also different online free courses that individuals can take. You could suggest to him to think about learning a foreign language. There are programs where he could work w/ a foreign student teaching him English while the other one is teaching him his native language.
If he is to use the computer for extended periods of times, you can ask him to use it in a productive manner. Of course, it would still be good for him to have things to do out of the house as much as possible.