I'm studying for the 6/15 MCAT now. Perhaps I'm being a bit ambitious, but I'm trying to divide up all the passages from the TPR Hyperlearning science workbook over this month. That probably means about 3 passages each day from bio (83 passages total), phys (60 passages), chem (91 passages), ochem (45 passages), and then 3 a day from EK's 101 Passages VR book. I'm not going to start ochem passages until I really study the EK Ochem book though.
Sad thing is, I get almost every Bio question right, I usually get 0-2 questions wrong per VR passage, but Chem and Physics are a nightmare (Chem moreso). Yikes.
I'm also doing as many Chem and Phys questions from the 1001 series as possible, but really just focusing on the sections that I'm not scoring to0 well in on the AAMC practice tests. (I.e., topics that I got 0% to less than 75% right on).
I don't think there's any magic number to practice passages. Just do them until you feel more comfortable with the format (I figure it'll help me not fall for silly mistakes on the real thing if I make those mistakes while practicing), to help get used to applying the concepts to more unfamiliar material in passages, and to just help nail down the material. Good luck.