From the IRS website: For an EACA or QACA, the employer must notify all employees who are eligible to participate in the arrangement 30 - 90 days before each plan year begins. Here's a link.
https://www.irs.gov/retirement-plans/faqs-auto-enrollment-when-must-an-employer-provide-notice-of-the-retirement-plans-automatic-contribution-arrangement-to-an-employee