I don't understand why people have always said to use MMRS for Step 1. If you use it to understand things it's fantastic as it explains everything clearly. But as a board review book (i.e., high yield review), i think it's very inappropriate - at least that's been my experience.
I would really only use it as a reference. Or read it only if there's sufficient time before the boards.
I read it in five days, but wasted some time. Probably if I was maximally efficient could have read it in 3.5-4 days, but might not retain as much. One week later, I've forgotten a lot of the details, but hopefully some of that residual memory will stick around for the exam. It doesn't seem to emphasize the same points as FA.
I read it during M2 but not during board review. I simply didn't have enough time to read the entire thing in my month before the boards. I read FA and reviewed sections of MMRS. I think the real key is to do lots and lots of questions. I started out doing terribly on UW (like 30-40% in micro) but now i consistently hit 70-80% on micro and it's been one of my best sections on NBME.
I spent two full days (at least 12 hrs each day) reading MMRS and annotated the info into FA. And so far, it's worked fine for me. If you haven't used MMRS before, it may be difficult for you. I used it during my Microbio class during the year and loved it then, so I knew I was going to use it for Step 1.