The Stereotypical work day in the western culture, classically refering to the Mid-1950's conception of a father in a pressed suit working at some nameless company in some nameless office high-rise. It means, quite obviously, to arrive at work at 9 AM and then leave work for the day at 5 PM.
This of course makes the dangerous (and dated) assumption that your employer gives you paid lunch times and counts them as part of your day. I say dated because while that kind of idea sounds absurd now (A boss paying your to eat food? As if!), in talking to some of my older relatives who were of working age in the middle of the last century, I have been led to believe that 30-50 years ago it wasn't odd at all.. and in many cases it was expected. However in this day, many full time days start at 8 AM and go to 5 PM to make allowances for an unpaid hour-long lunch break.
This phrase also takes on a certain triteness in the new age of the global economy where business is happening non-stop all around the word, and especially in some Information Technology positions were the very idea of every leaving your office just seems like some fanciful dream.