Ever since starting graduate school I've tried to make scientific reading a part of my daily ritual; I track pages read using Beeminder, and the graph doesn't lie. It keeps me honest.
I aim to closely read and summarize 5 pages per day and skim a few other abstracts besides that. I spend about half my time looking at data and figures which doesn't contribute to my daily "page count." When I'm reading about a new topic these five pages can take several hours, but on topics I have more background in five pages might only take an hour per day.
I guess since everybody defines "read" in a different way it's hard to get an objective answer about how much reading is enough. How much people read seems like a bit of a sensitive topic among real-life colleagues because everyone has a bit of anxiety that they aren't reading enough. But for those further along in their academic path, I'd like to hear how you approached the literature early in your graduate school career and what you think is a sufficient amount.
I guess this all distills down into two main topics:
When deciding what to read each day, should I focus on depth or breadth?
Is five pages of close reading per day enough? I know it doesn't sound like much, but it takes significant mental energy to meet that goal. And consistently reading 5 pages per day adds up to a lot over time.
Edited to add: I mostly read about petrology, volcanology, structural geology, and tectonics if that makes a difference. By "page" I mean "page of text" so if I'm reading a structural geology paper with lots of maps and figures I discount for those and a "ten page" paper becomes a 5 page paper for my purposes.
This is about my experience in applied atmospheric physics
As others have said, it is often reading for depth that is more effective. Making sure that you understand the key concepts and connections made between the main concept and the subsidiary topics within a paper. Usually, I read the Abstract and Conclusion to garner the main points of what the paper sought to discover, what method they used (established, modified or new) and what were the overall results.
In many of the papers that I have read (and written), the method is often in stages corresponding to specific results in the results and discussion section (they are often merged in my field) - I tend to read the method, derivations therein and the results of each stage - which means flipping between each section. All the time taking notes.
At times (not very often), the method stage makes reference to another paper, so that paper is retrieved and worked through in the same way.
I have found this process to be quite quick, as each stage comprises a short paragraph and equations in the method; a corresponding sentence in the results, often with a table or graph and a sentence or two of explanation.