How much time to spend reading?
When reading, focus on the larger concepts. Skim. If the passage starts talking about details like years or book titles, skip it but note where it is mentally so if a question does ask you about a specific detail, you know exactly where to look (and then when you spend time on that section, it's definitely worth points). A great way of developing VR skills is to stop after each paragraph and do summarize in five words WHY the author included that paragraph. Not WHAT is in the paragraph, but WHY. As in, why did he talk about that point? What did it do to his overall main idea? Support? Reject? Expand? etc.
Your job isn't to know every single word that was talked about. It's to know the main idea (and tone) of the passage as a whole and the purpose of each paragraph. If you know those two things, you're going to do very well on that passage because that's what AAMC tests you on.
For instance, if I'm reading a passage about Confucious.. I'm not spending time reading about how he grew up or what year he published his books or whatever. When that comes up, I skip it. Only reading to get the main point of each passage so I don't care what the details are.
There are too many details to bother with. There's only so many questions and odds are, that one detail you spend ten seconds reading about won't be asked about. When you quickly skim or skip those parts, just remember where they are. So if you are asked about it, you know exactly where to look and can then spend the time to read closely (when you are assured points are at stake).
For practice, I'd suggest stopping after each paragraph of a passage. Summarize to yourself in five words or less WHY that passage was included (not WHAT was in the passage but WHY the author talked about that content). Write it down (I did this during the real thing as well, pencil in hand [active reading] is the way to go). You'll have a nice sentence summary of the passage afterwards as well as an indicator as to where to look later on for details/follow-up. You'll also focus more on the main point this way.