I have seen the Migne collection, and I concur, it is hard for me to believe that such a human feat is possible. That being said, just look at the amount of literature Orestes Brownson read. He considered himself a book reviewer and it was nothing for him to review multi-volume theology works. The amount of literature Brownson read is staggering and his Brownson's Review (1844-76) is some 20,000 pages. That said, we moderns waste a lot of time. I believe there are 300 volumes of the Latin fathers; if Newman spent 20 years reading the Fathers, this would be 15 volumes a year, roughly one a month, which seems incredible to me, but theoretically doable.
Perhaps that was just a different time when people could just sit and read. I would think a Cardinal, however, would be too busy for that, or even a simple parish priest would be.
So one Migne volume in a month, that to me would be incredibly daunting. They're huge volumes with tiny print, and, what?, probably at least a thousand pages each, right?
Perhaps he meant that he's read FROM all the Fathers. I was in graduate school at The Catholic University of America in Greek/Latin and finished the coursework for the Ph.D. ... though didn't believe I could spend another 7 years between the two rigorous exams they had and the dissertation. I went there precisely because they had the only Ph.D. Classics program in the country that emphasized Patristic/Medieval Latin and NewTestament/Patristic Greek. Unfortunately, however, their requirements were obscene where you had to do a list of Classical texts in Latin / Greek that were AS EXTENSIVE as any other Ph.D. list in just straight Classics, for the Master's degree, and THEN you had to do an EQUALLY gigantic list in the New Testament / Patristic / Medieval area, not to mention art, archaeology, epigraphy, paleography, etc. In that extensive time period, from the 2nd/3rd century B.C. (due to needing to know about the Septuagint for the Greek side), until about A.D. 1200 (or so), the amount of material is absolutely overwhelming. There are so many different periods and locations throughout the world that it's mind-boggling, and so IMO their program was WAY too broad, and I'm not sure who the heck could ever finish it without intending to become a Professor (there) after about 7 more years after coursework ... and that's assuming you don't need to work for a living during that time.