So, you have to put in eight hours of actual work? A usual workday is eight hours, but I'd say most employees probably do less than four hours of actual work every day.
Are you a freelancer or an employee, if you don't mind my asking?
I am an employee at a large distribution company for IT hard and software. I think there are many factors that play into how long a productive day really is:
Type of work - is your job requiring brainwork all the time or maybe you have some work in-between that just requires execution prime to thinking work?
Environment - how good are you shielded from distractions?
Communication - do you really need to be shielded? Maybe you need a steady communication to proceed with your work. I'm thinking about the guys at stock market e.g.
Type of breaks - This is a big one and up to personal preference to a degree. Are you forced to take a break? Is it a hard force or a soft force (telephone vs. mail). Do you have to respond immediately? A big problem for me e.g. is interruptions when i'm in the middle of it. It takes a fair amount of time to get back into the zone.
If this happens i often like to refer to this article: http://blog.ninlabs.com/2013/01/programmer-interrupted/ It's hard for people to understand why interruptions can be very harmful for work.
So if i say 6 hours, this is the maximum for me to work concentrated on one topic. Interruptions diminish this number, shorten my concentration timespan and often make the day not as productive (subjectively).
edit: formatting