I'm not certain that you can but I have purchased both SmartPad and Pocket Informant apps, both of these allow you to set working hours although I think all hours are visible in all of them but I'll have to check later.
No way to do it in the stock Calendar app, at least not that I've ever found. On the good side it always opens with the current time marked and visible, so you don't have to scroll down or up for what's happening int he near future.
