How long should I work before taking a break? I'd like to work as effectively as possible for 8-9 hours a day, whether at work or studying.

At some point in completing my major in psych I remember hearing about the importance of taking breaks while doing work (e.g., Trougakos and Hideg, 2009 on momentary work recovery). Gopher et al (1995) discuss  ultradian rhythms and working for 90 minutes for every 15 minutes of break. Some methods like the pomodoro technique suggest 25 minutes of work for every 5 minutes of break, with a 15 minute break every 4 cycles.


Is there recent, widely-accepted research on what work-break rhythms work best? (Even if the research was done for specific tasks or for specific populations.)
How does the optimal work-break rhythm vary by task, context, or other moderating factors?


References


Gopher, D., Lavie, P., &amp; Zomer, J. (1995). Ultradian Rhythms in Prolonged Human Performance.
Trougakos, J. P., &amp; Hideg, I. (2009). Momentary work recovery: The role of within-day work breaks. Research in occupational stress and well-being, 7, 37-84.
It appears extended working days, such as, 16 hours are counterproductive. 
(1) Safe working hours - doctors in training a best practice issue examines the effects of extended working hours on performance for doctors. It is recommended that a 7-8 hour working day is more productive than extended days. One issue being the sleep deprivation that accompanies extended working days.

There is research on optimal work-break rhythms. It has been found that work performance is optimized  in 90 minute time slots. This is improved by dividing this into 25 minute time slots, with short breaks. With relation to the type of work, it is suggested that breaks be taken away from the working environment, optimally, within an altogether different environment. 

This is consistent with natural ultradian cycles:

Image taken from The origin of the 8 hour work day and why we should rethink it a blog by Leo Widrich

I have quoted extensively from the following article:
(2) Best Rest Practices for Optimal Productivity and Creativity.
This addresses your questions comprehensively and also confirms that an extended working day is counter productive. 

Timing of natural working rythms


  Rossi champions the 20-Minute Break in his book of the same name for
  peak performance. And Anders et al studied in 1993 a group of
  violinists and discovered that the best performers practiced in
  90-minute intervals and learned not to over-practice.
  
  We can break down that 90-minute rhythm even further into 25-minute
  spurts. Most people can pay attention to a talk for 25 minutes (hence,
  the brilliance of TED Talks’ 17-minute limit).
  
  Hence, another client, a highly reputed novelist and short story
  writer, wisely writes in three 25-minute periods—each divided with a
  timer. At the end of the third period, she takes a walk or switches to
  another activity


....

Breaks away from the workplace


  I recommend to clients they remove themselves from the work
  environment if possible.
  
  For a corporate client, I recommended he schedule a walk to another
  part of his vast office quarters, glance out a floor-to-ceiling
  window, and then return to his office room, sit in a comfortable sofa
  normally reserved for clients, and remember the last time he was
  outdoors working on his ranch—one of his favorite activities.
  
  Step outdoors if you can. I suggested that one client who gets
  overwhelmed with multiple tasks in the afternoon to step outdoors and
  sit in her garden for fifteen minutes and simply observes the colors,
  textures, sounds, and smells. Ideally, she would weed for fifteen
  minutes as the physical activity more likely will let her processor
  mind incubate some of her creative problems.
  
  Or step away from your work space, and read four pages from a book
  that brings you delight.
  
  If you can jog or bicycle for 20 minutes, even better—as you’re also
  increasing the chances of your hippocampus forging new neurons in your
  middle-aged gray matter, according to Fred Gage of the Salk Institute
  and other researchers who’ve examined the benefits of intentional,
  enjoyable cardiovascular exercise.


...

Techniques to combat fatigue


  DISTRACTION CAN RE-BOOT LONG-TERM FOCUS. () Here’s a mind-twister:
  Atsunori Ariga and Alejandro Lleras of University of Illinois at
  Urbana-Champaign, United States questioned the assumption that loss of
  focus came from, say, fatigue or lack of attention. In 2010, their
  study published in Cognition concluded that a deliberate distraction
  or introduction of a second task actually can increase vigilant
  attention on the first task.


....

Utilising multi tasking to break mental fatigue.


  Pay attention to when you need to introduce a quick second task. Maybe
  sending off an email or text message will free up mental bandwidth and
  get you re-committed to the high-thinking task. Do this in tandem with
  the “Break with Rhythm” suggestions above.


....

The benefits of a daily nap


  Some students took no naps. Some students took catnaps. And some
  students took full-fledged 90-minute naps complete with REM
  dreams...//...    students who took the full REM-level naps showed
  boosts in creative problem-solving performance.


....

The concept that a longer working day is counter-productive


  Schedule non-digital time in the evenings, especially 45 minutes
  before sleep. Set up a bi-weekly schedule of evening rhythms: One
  Monday as “reading night,” One Tuesday as “date night,” et cetera. If
  you must work some evenings, schedule work evenings. Make them the
  exception instead of the rule.


 (1) Safe working hours - doctors in training a best practice issue
ANDREW LEWIS
Andrew Lewis is Senior Industrial Relations Adviser with the Australian Medical Association Victoria
(2) Best Rest Practices for Optimal Productivity and Creativity
How and when to break from creative work.
Published on April 30, 2012 by Jeffrey Davis, M.A. in Tracking Wonder
The Seattle Longitudinal Study  K. Warner Schaie, Ph.D. and Sherry L. Willis, Ph.D. UW Medicine
Life In The Middle  Life in the Middle: Psychological and Social Development in Middle Age
 edited by Sherry L. Willis, James B. Reid
Recovering from Work: Interview with Sabine Sonnentag 
Brief and rare mental ‘‘breaks’’ keep you focused: Deactivation
and reactivation of task goals preempt vigilance decrements
Atsunori Ariga, Alejandro Lleras
University of Illinois at Urbana-Champaign, United States
 The Role of Deliberate Practice in the Acquisition of Expert Performance
K. Anders Ericsson, Ralf Th. Krampe, and Clemens Tesch-Romer
Psychological Review
1993, Vol. 100. No. 3, 363-406
Copyright 1993 by the American Psychological Association, Inc.
0033-295X/93/S3.00