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.