For most schools I believe it is 3 years. Now, it varies from school to school as to from which date they consider the 3 years. Some take it from matriculation, some from application.
In my personal experience, I had to retake the MCAT with a previous score of 34 from 2005. However, it was funny to see how many interviewers were not aware of this requirement. I had to repeatedly explain why I had taken the MCAT twice (especially since I did 1 pt less in Verbal the last time)😳.