I can only speak for my optometrist - he works 3 days (27 hours) a week. The OD that works for him works about the same, I believe, but also works at another office part-time. He works beside a Pearle Vision, but he owns it and the building that he works in, so I guess that would be considered private practice?...
My OD is a farmer on the side, so while he's not in the office he's tending to his farm
(he's a goat breeder)