Today I am sitting on my desk with a pen and a notepad next to me and writing down everything I do and the time it takes me, so I can understand why I keep reaching the end of the day and not move forward with what I had planned for the day.
So, my conclusion is: on average for every hour I spend "working" 20 minutes are lost. From breaks, phone calls, random searches I do online, etc.
So, on an 8 hour work day, just around 5 are actually productive.
@hugo thing is, tho, we need a break every now and then so we actually keep being focused enough and healthy, so those breaks still count anyway. 
@hugo see? So, those breaks are essential for work, too. Thus, they count as productive time too. Just a different experience of what it means to be productive.