This varies wildly, I'm not sure anyone will be able to answer. Even if they do, it often depends as much on WHAT you are reading. I can breeze through 50 pages of ethics textbook in no time at all and spit back a 5 sentence summary. It might take me an hour to read through a 6 page paper on "wet" neuroscience, and still sound like a 5 year old trying to explain particle physics. Some articles I can skim, get the gist, and that's all I need. If its something that's interesting to me, or very important (i.e. models that are the foundation for much of our research), I might go through it several times, etc. Most of this goes for writing as well. I can bang out a stats homework assignment in 30 minutes. My thesis proposal? We're talking calendar rather than clock. Others I know can write really fast...I'm usually one of them, but I got horrendously stuck for a very long time on my thesis.
Really what it boils down to is, you are never done. No one knows all the literature in their field. Even the most well-known, well-respected senior faculty members will have absolutely astronomical gaps in their knowledge. Its just the nature of the beast. Most people cannot keep up with the reading in THEIR area anymore, let alone other areas.
In other words, the work is never done. You just have to do as much as you can, and set limits for yourself based off the kind of balance you want. Getting the bare minimum done isn't hard, I wouldn't worry about it. Beyond that, its really up to you.
