Difficult to tell. Our most highly respected individual contributor in our department comes in at 11:00, leaves at 5:00, and basically works on whatever catches his fancy whenever he pleases (Which, thankfully, usually results in a reduction in labor for our and other departments each time he releases some new code/product)
When particular tasks are placed on his plate - he is so overwhelmingly competent, that he simply nails them and moves onto the next task. The goal is to get _very_ good at your job so you do _less_ work.
And he is easily the most popular member of the team. People love him to pieces and I've never heard anyone suggest he isn't carrying twice the load of everyone else.
As I tell my team - Our Goal in life is to become so proficient at our jobs that we only need to come into work for an hour a day. If we develop our level of excellence, or ability to automate, and our ability to deliver to that level, I'm more than happy to have them in the office for as little time as is required to complete the tasks of their position. (presuming that the median person with those responsibilities would take 40 hours a week, of course)
As it turns out, with the exception of Director Level employees, the most highly paid _contractor_ in our company works for precisely 2 hours, from 9:00 - 11:00, each day. They are available 24x7 for assistance, but, if they do their job properly, they rarely are called in for help.
That is where I think we want to be. That should be our goal. Spend our time with family, in nature, pursuing those objectives which bring us inner happiness.