I had 3 MMIs and was accepted for one of them so far. Might sound like overkill, but I spent maybe 5 hours before my first one (pretty spread out though, and included a practice session my dorm put up + prep for general interview questions), an hour before my second one and maybe half an hour before my third. I spent my time coming up with a process to follow and doing practice questions. Interview days can be draining and the last thing you want to be thinking about during the tour is that you wish you had extra practice. Of course do not try to plan for every contingency through. The point of practicing for me was to feel comfortable and prepared enough to react to whatever changes happened. You'll MMI with stoic people who stare at you like a spider and chatty people who talk through half your interview time and you should be ready for both. You also have to take into account your situation-- you'll need more practice for interviewing if you're an introvert applying straight through than if you're a really chatty person who's already held two jobs, for instance.