I recently came upon research showing a normal adult attention span to be 20 minutes maximum before needing a short mental reset. The author of the information below has found that 40 minutes of focused-attention creates the most accomplishments, followed by a short period of collaboratively-focused attention, followed by a break.
http://www.5min.com/Video/40-Minute-Focus-For-Breakthrough-Results-at-Work-517067674
http://less-is-more.typepad.com/files/the_results_curve_tm_ebook.pdf