As far as time goes- I suppose that depends on you. For me it's been ~3-5hours/week outside of class, but I've had some exposure to the concepts before and I'm pretty good at memorizing (lotsa dirty mnemonics😉- seriously though, study smarter, not harder. If it makes it stick in your head, for whatever reason, then it's a good mnemonic.). If you know memorization will take you longer, you could ask the prof now what large chunks of info you'll be responsible for and start over the winter break.
The concepts that seemed to give most of my classmates problems fell into 2 categories: memorization and math/gen chem skills. Some examples (which might differ from course to course):
We've had to memorize so far: functional groups (which you should have from orgo, but everybody seemed to have forgotten), amino acids structures, nucleic acids structures, common carbohydrate structures, and coming up are glycolysis and citric acid cycle pathways with structures.
Math/gen chem stuff: relationship between pH and pKa (how to calculate either and then use to make predictions about rxns), Michalis-Menton equation + Lineweaver-Burke plots (being able to use it and understanding what it tells you), deltaG (understanding, calculating, using to predict rxns).
None of these seemed particularly difficult to me, but tripped up a lot of my classmates that were either seeing them for the first time or that had apparently forgotten everything they'd ever studied prior to the current semester.
Mostly though it's about chemical logic. Which if you understand that well enough from orgo to be getting an A, then you'll probably be fine with biochem.
As for physio or anatomy, I haven't taken them, but they seem to be about the same level of time and memorization as biochem. I'll let other people comment on usefulness for MCAT, since I haven't taken that either. (though from the practice tests I've taken, the biggest topic I've seen from either of those seems to be the kidneys, which you could maybe learn on your own)
