Q.  How long does a [Michigan] township need to retain FOIA documents?  

A.  The retention of Michigan records is governed by approved schedules issued by the Department of Technology, Management and Budget (DTMB).  State government agencies are required to maintain a FOIA file for one year after its creation.  See General Schedule #5.06.  The General Schedule #10 for Michigan Townships does not appear to include an agency-wide retention period for all FOIA files.  The retention schedule for FOIA files for Township Clerks and Township Treasurers, however, are both one year from date of creation.  Therefore, I suspect that the same period is likely to apply to your agency.  To confirm, I recommend that you contact DTMB's Records Management Services at (517) 335-9132.