I think the average person's capacity for deeply focused work is generally considered to be around 6 hours per day. I've found that to be the case for me as well. However, I can be maxed out at my day job, but go home and work on an unrelated side project for several more hours.

I highly recommend Cal Newport's blog as he writes phenomenal content on "deep work", the importance of focus, and how to maximize your "deep work" output. --> http://calnewport.com/blog/about/

Separate out "deep work" (design, coding, problem solving) from "shallow work" (email, social media). Do as much deep work as possible every day, but then fill in the rest with shallow but necessary work.

P.S. My average day-job working time is 8 hrs.