As a workaround people could set recurring appointments during non-work hours so it would show they were unavailable. For instance, I don't want to show I'm available before 10am each Wednesday, so I would set up a recurring appointment to show as busy form midnight-10am and recur on Wednesdays. They would have to do this for days and one in the am and one in the pm, but it would work.
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Bug 15427 – Ability to set working hours on a per-user basis