For every 8 hours at work, how much time do you spend doing nothing useful?

[The Fury]

I work 8, in that 8, I'd say I do maybe 4-5 hours good work. The issue is though, it used to be 8 plus more a day, we were short staffed, over worked and every day I did overtime. The work load has increased but our methods and working process has decreased time. I've got better at my job making my work time for things that would take me 2 hours, only 30 minutes for example. Plus it's not consistent work, so one day I'll be non-stop work, the next day 2 hours if that.