When To Take A Break

Found this article today on Lifehacker.  It really make sense.   Everyone should read it.
Source: Take Vacations When You’re in the Middle of a Project, Not When You’re Finished.
 
Edit:
Also check out this one – The Trick to Accomplishing More by Doing Less: Take Breaks
It talks about how things that take longer than 90 minutes with no breaks are less productive due to blood glucose levels.