Security deposits are equal to one month's rent and are due at the time of move-in. They are then held—and cannot be accessed—until you vacate the apartment according to University procedures, which includes returning all keys and having the apartment inspected for damages.
Interest payments on your security deposit, if applicable, are sent out to the apartment address on an annual basis. An IRS 1099 form will be issued for any interest income.
A security deposit is not your last month rent payment. It is the policy of Columbia University that rent must be paid through the day of vacancy. Refunds take up to 60 days to process.