I was assigned about a dozen exercises from Fux as a Sophomore at St. John's. I remember them going very quickly, maybe ten minutes each, and so thought I could tear through the sixty two-voice exercises in a week. It's actually taken me about six weeks. Why?
I think there's an interesting interplay between my workflow and my standards of success. When I did these as a 19-year-old, I worked on paper in a dorm room. I'd usually show up for class a little early so I could play my exercises on the piano. (I've never been able really to read music, so the only way I can know what they sound like is to play them.) Every time I played one, I was impressed that some part of it sounded neat. I was following these rules, marking down notes that I couldn't hear, making nice-looking patterns on the page, and when I played them, some part always sounded nice.
Now, as soon as I finish a first pass, I do it up in my score editor and have the computer play it back for me. It's still the case that some parts sound nice, but it's always the case that some parts could be nicer. So I fiddle with it, which is easy to do. I move some notes around on the screen, then hit "play" again, then fiddle some more. So, instead of being happy that some bit of it sounds nice, I keep working on it until I'm basically satisfied with the whole thing. This can take a long time-- I spent three hours on one of the fifth species exercises. Because tweaking my work is much easier now, I'm much less willing to be satisfied with mixed results.
The new way is probably the better way to learn, but it's definitely made the project more daunting than I thought it would be. I hope I have the stamina to make it through all 90 of the three-voice exercises. I'd love to do that. But I don't expect I'll do more than a handful of the four-voice exercises. Unless I somehow get much, much faster, there's no way I'm going to do all 120 of those.