For my class, we got 2 weeks of time after our finals. Then you can take another month to study, but it uses up your vacation month in 4th year (so 4th year will be extended til graduation time (May)). Don't forget about ACS - it's a month of basically half-days and you get the afternoons to study/do whatever you want.
TLDR - 2 weeks minimum. 6 weeks if you use vacation time. 10 weeks if you use vacation time + ACS month.
PS. I've had classmates who did the 2 weeks minimum, cause they wanted their vacation time at the end of 4th year. Several of them regret it, since their Step 1 scores were below average. Just a word of caution!