Dear Bernadetten, 

 Attention span, as measured by sustained attention, or the time spent continuously on task, varies with age. Older children are capable of longer periods of attention than younger children. Fatigue, hunger, noise, and emotional stress reduce the time focused on the task. Common estimates for sustained attention to a freely chosen task range from about five minutes for a two-year-old child, to a maximum of around 20 minutes in older children and adults. Internet may dcrease the teeagers' attention span. It is probably not possible to tell what the average attention span is in teenagers since it is an individual parameter but you can test a group of them. Here are papers with some ideas.

Turning into digital goldfish, BBC News article on how the internet affects attention span. 22 February 2002. Retrieved October 23, 2008

Differences in memory functioning between children with attention-deficit/hyperactivity disorder and/or focal epilepsy. Lee SE, Kibby MY, Cohen MJ, Stanford L, Park Y, Strickland S. Child Neuropsychol. 2015 Jul 8:1-22.

Attention and working memory abilities in children treated for acute lymphoblastic leukemia. Ashford J, Schoffstall C, Reddick WE, Leone C, Laningham FH, Glass JO, Pei D, Cheng C, Pui CH, Conklin HM. Cancer. 2010 Oct 1;116(19):4638-45.

Health in a 'post-transition' Australia: adding years to life or life to years?

Begg SJ. Aust Health Rev. 2014 Feb;38(1):1-5

Measuring working memory is all fun and games: a four-dimensional spatial game predicts cognitive task performance.

Atkins SM, Sprenger AM, Colflesh GJ, Briner TL, Buchanan JB, Chavis SE, Chen SY, Iannuzzi GL, Kashtelyan V, Dowling E, Harbison JI, Bolger DJ, Bunting MF, Dougherty MR. Exp Psychol. 2014;61(6):417-38.

Measures of digit span and verbal rehearsal speed in deaf children after more than 10 years of cochlear implantation. Pisoni DB, Kronenberger WG, Roman AS, Geers AE. Ear Hear. 2011 Feb;32(1 Suppl):60S-74S.

Effects of the Visual Exercise Environments on Cognitive Directed Attention, Energy Expenditure and Perceived Exertion.

Rogerson M, Barton J.

Int J Environ Res Public Health. 2015 Jun 30;12(7):7321-36.

Dear Bernadette,

Despite several attempts, the jury is still out whether attention span has a fixed value for every student, in every situation and every time of the day or not. It is extremely variable. I would like to copy-paste a section of the report that I read online a few months ago. I searched for the article and it is still online. Perhaps that might help you.

(Source: Review of Bunce, D. M., Flens, E. A., & Neiles, K. Y. (2010). How Long Can Students Pay Attention in Class? A Study of Student Attention Decline using Clickers. Journal of Chemical Education, 87, 1438-1443.)

This work produced three main findings. First, the researchers observed that the most frequently reported length of attention lapse was 1 minute or less, suggesting that very short breaks in attention are more common than longer breaks. The second main finding was related to the frequency of reported lapses. If it is true that the students typically have a 10 to 15 minute attention span, one would expect to see a pattern of reported lapses every 10 minutes or so, but that was not what the researchers observed in this study. Instead, across the three courses that were studied, the researchers observed a pattern in which the first spike in reported attention lapses occurred just 30 seconds into a lecture segment, likely reflecting the same “settling-in” period of disruption observed by Johnstone and Percival (1976). The next consistent spike in reported attention lapses occurred at 4.5 to 5.5 minutes into the lecture, followed by another spike at 7 to 9 minutes, and then another at 9 to 10 minutes into the lecture. This waxing-and-waning pattern continued throughout the lecture, with attention lapses occurring more frequently as the lecture progressed. By the end of the lecture, lapses occurred about every two minutes.

The complete article is available at http://teachingcenter.wustl.edu/Journal/Reviews/Pages/student-attention.aspx

While this might not help your focus on "teenagers," I would also look into behvioral characteristics of "Generation Y" (also known as "Millenials"). 

Haven't looked up the references above, but should I assume that no one has done an attention test/task in a representative, population-based sample to produce good population estimates? Who needs a dissertation topic!? :)

I read somewhere that it is about 10-15 minutes.