green cleaning term

I received a comment on a design phase review that since this is a core & shell building, the owner has to either include green cleaning in the lease, or agree to control the cleaning for both the base building and the tenant spaces. We can't do a lease agreement, since some tenants have already been signed, so I will ask the owner to control cleaning for the entire building, including tenant spaces. The question is, how long must the green cleaning contract last? If I am obligating the owner to clean the entire site, I want the contract to be as short as possible, but I assume the LEED reviewers will not accept anything too short (like month-to-month or whatever). Is there any specific requirement in the credit as to how long this contract must be? Has anyone had any experience with a contract term for this credit? Thanks!