Columbia University has overarching policies on voluntary, involuntary and military leaves of absence:

http://facets.columbia.edu/voluntary-leave-absence-policy

http://facets.columbia.edu/involuntary-leave-absence-policy

http://facets.columbia.edu/military-leave-absence-policy

Mailman School Policy:

Students who will not be registering for more than two semesters must apply for a formal Leave of Absence. This can be accomplished by completing the Leave of Absence form available in the OSA and online at:

Students must meet with their advisor, secure his or her signature and return the form to OSA. Leave of absence requests are approved for one year. Exceptions will be considered on a case by case basis through a petition to the student’s department and final approval by OSA. Students on leave of absence for more than two years (cumulatively) must reapply to the school.