I took 3 weeks off in October to study for it, since my schedule allowed for it - but I did wedding planning and was trying to schedule interviews as they were coming in (which was more stressful than I'd anticipated). That was probably a little more time than I really needed -- 2 weeks is plenty if you're efficient.
Otherwise, I have plenty of friends who did it at the end of a 4 weeks elective, during which they were working "normal" hours on weekdays and had weekends off. I found it was hard to study continuously for long periods of time, anyway. Good luck!