I completed the google project management specialization in a month while working 30 hrs a week. I don’t recommend it, but I did it. I am also a serial note taker and managed to take down somewhere in the realm of 200 pages of notes, including screenshots. So that makes it a little slower.
Needless to say, at the end of the month my brain was fried. I literally worked and studied and nothing else. The last course in the specialization was a bit of a blur. I wasn’t really retaining much by that point.
It really depends on the course. I did the Google UX design course and there was no way I would ever be able to complete that in a month. That was a solid 3 months.