Working in an enterprise environment, and currently working probably 8-9 hour days, I think I can shed some light on this.

All hours between 10 and 6 are basically unproductive hours, because they get taken up by useless meetings and interruptions.

Any extra hour I work outside of that time, is worth basically 3 or 4 inside it. So working 10 instead of 8 hours practically triples my output.