The best, and yet most useless answer I can give is "it depends."
But if I must give you a number:
For someone, who did well in all their classes (let's say above the mean) at a medical school with a good curriculum that is, however, not a completely boards-oriented, I'd go with 5-6 weeks for a solid preparation. They'd probably need 3 weeks to review stuff, and then 2-3 extra weeks to learn what's not been taught in their school (there's always something) and to get some lower-yield items.
As a rule of thumb, I'd never go with less than 3 weeks (regardless of how good a memory you have) or more than 7 weeks (that's what I am taking, and my last week is completely useless, since I am burned out/don't care anymore/spend more time posting messages on this forum than studying).
I'd absolutely NOT take a year off, just to do some kind of light work + study. Even if one does really bad in his/her classes, as long as he/she passes, it means they know a lot and are likely to forget more than learn in that one year. Remember, there is no such think as a "bad medical school" (at least in the us), so passing it's classes is a sign you are being well-prepared. (That's my guess...maybe someone who actually did that will think otherwise).Anyway, hopefully more posts will follow, so you can get a better picture of what people think.