Hi Jay,
In very broad terms as it depends on the material, I suppose I spend 30 minutes to an hour listening to and processing the first track in a project, with perhaps 15-30 minutes per song thereafter - some go quicker than that, some slower. Once I've got the feel of the thing as a whole I may come back and tweak the first song or two in light of my understanding.
To me 5 hours per song is complete overkill: that's maybe a week for a typical album! No commercial mastering establishment I know of could afford to take that long, they'd speedily go out of business; more importantly, I don't think it's necessary if you have reasonable grasp of what you're doing. I do remember a chap called Bill Roberts claiming time taken of that order some years ago, but I don't believe he was in what I would think of as the mainstream of mastering work.