I came across a compiler bug when doing the course with gcc outputting incorrect ASM which was quite exciting as I've never come across one in the wild before. I really should check if that's been fixed and report it if not.
One thing I would say is that it was a fairly linear amount of learning, so even if you only complete part of the course you can learn a lot.
One of the requests on the course was to include time taken for the assignments as part of the submission. This was part of the (locally run) grading script, so even though I didn't submit I still completed it, luckily.
There were 11 assignments, and each took 5-10 hours apart from a few I got stuck on - I've got one assignment listed as being 30 hours (and I still remember the frustration. I nearly got to the point of emailing the course creators to ask for help but figured it out in the end as part of it was my aforementioned compiler bug).
On top of this there are two lectures a week (often ran over time with questions) so about 3 hours per assignment and some reading material.
I had used C before this course and was very rusty. This caused me to make some mistakes in the beginning, like pulling in standard libraries rather than using the ones as part of XV6, that someone new to C would likely not have made.
So in total I'd say 36 hours of lectures, 10 hours reading, 100+ hours doing assignments.