We are choosing to revoke your ability to bank with us effective (5 days from today) date, please come in on that day, if not sooner, to collect the balance in your account or we will mail you a cashier's check for the balance.
I opted for the 30-day limit because the US Treasury requires a 30-day notice if any government benefit is being provided. They do all for an immediate closure for fraud, but not sure this instance has been definitively proven as fraudulent, based on the original post. if not government benefits are going to it, then i would say immediately.
From the US Treasury Green Book:
Financial institutions may close an account to which benefit payments are currently being sent
thereby revoking the enrollment authorization by providing a 30-day written notice to the recipient
prior to closing the account. In cases involving fraud, accounts may be closed immediately. The
financial institution cannot revoke the enrollment authorization by notifying the federal agency and
not the recipient.
The 30-day written notice should remind the recipient to make other arrangements for the
handling of their payments. The financial institution must credit to the recipient’s account any
payments received during the 30-day notice period. The financial institution must also immediately
return to the federal government all payments received after the 30-day notice period. A financial
institution that closes an account without properly terminating the enrollment must make the
funds available to the recipient until proper notice is provided.