It really depends on how much of the content or expression I care to absorb. For example, it's taken me about a year to finish half of A Thousand Plateaus partly because I take rigorous notes and find the implications so reeling that I have to take breaks from time to time.
Ulysses took me about two weeks, but I was spending hours per day engrossed in it, meaning that the jump from everyday conversation to the language in the book wasn't happening; the better primed you are for a genre, it seems, the easier it is to trudge through it. I definitely think it was a milestone, not only in my reading, but in my attention span. I'd guess an 80% comprehension of content, but the expression was just too good to let go of.
Currently I'm reading Nietzsche and Philosophy. If my estimates are right, I will have completed it in ten days. Since I'm familiar with the style of the author, it's easier to plan and think ahead and to find the main points. This certainly helps, particularly in fiction that is formulaic (The Magic Treehouse series, one that I read in my childhood, comes to mind).
But again, on priming: I think that, on one hand, if you read a similar (fuzzy) genre in sequence, it makes it easier to critique and look into the details. On the other, everybody has their limits. For example, reading ten non-fiction books on climate change written for a general audience will at some point become too repetitious to bear. In my view, it is best to approach a book as you would a transit map, in order to see what line you should transfer to, or what stop you should get off at. Selective reading is good. Reading to put yourself to sleep is pointless.
Specifically on your questions:
1) Brave New World took me a day, but a similar length book, The Uses of Pessimism took me six. One was made to be easy to digest, the other was challenging in terms of my own ideology.
2) Reading books is not a competition. You can read any number of books. My mother has a collection of Steven King books in the dozens, but there is not the intellectual challenge, I think, in the fewer books that I read.
3) I think it is best to view reading (so be it, lists) as a demarcation of your literacy. The CIA World Fact Book claims a 99.6% literacy rate here in the US (last I checked), but that statistic does not consider the depth of field of the recognition of letters; it may only consider basic, basic understanding of the alphabet and how to pronounce words. It other words it is the meta-linguistic knowledge of what to gain from a book, versus how to merely sound it out, that is the true literacy.
If you are interested in what I'm saying here, I would suggest this book: How to Read a Book