Other than at time of hire, the employer is to provide the new Form W-4 to any employee who wishes to make a change to his/her federal income tax withholding. The other required time is when the employee had claimed Exempt on their previous Form W-4. Under IRS regulations, when an employee has claimed Exempt from federal withholding, that exemption status is valid only for the tax year in which it was claimed. The employer must provide the employee with a new Form W-4 to complete no later than February 15 of the new calendar year.
See additional guidance here or at: https://www.irs.gov/taxtopics/tc753.html
(January 2020)