How long do people have to get their belongings from a storage locker I pay for?

Michigan Law has specific standards Check out MCL 570.521 et seq. Michigan Compiled Law Act 148 of 1985 SELF-SERVICE STORAGE FACILITY ACT http://www.legislature.mi.gov/(S(2y1umbipaxsju055w0eqkyql))/mileg.aspx?page=getObject&objectName=mcl-Act-148-of-1985

You need to serve the tenants at all of their known addresses and any emergency contact information you may have, with a notice of belief of abandoned personal property. If the storage area is not locked, you need to inventory the property and list it on that notice. Following expiration of that notice, you may keep the property or dispose of it in any manner you desire come if it is reasonably worth less than $750.00, And otherwise follow the statutory procedures for the sale of the property and the disposition of sale proceeds. If you choose to treat the property is worth less than $750, you should keep pictures of the property so valued for your records, in case that decision is contested.

I would see an attorney, and determine how exactly to attack this issue. You do not want to be in a position where they come back and claim that you somehow are responsible for the loss of their property.