a day of work should be what your employer asks of you.

Most people don't do more than 5 hours work per day.

But why measure in time over productivity?

what do you mean by this?

Sales people are rewarded for productivity and not directly constrained by hours, right?

Work smarter and not harder and all of that...

oh well if you are productive enough then you'll get promoted and you can pick your own hours

or if you don't get promoted

you can go into business for yourself and you can pick your own hours