The general rule is that an employer may ask for recertification no more often than every 30 days while the employee is on leave. If the medical certification indicates that the duration is expected to be more than 30 days, recertification cannot be requested until expiration of that duration or six months, whichever is sooner. An employer may request recertification in less than 30 days if the employee requests an extension of leave, if circumstances change significantly from the prior medical certification, or if the employer receives information that casts doubt on the employee’s stated reason for leave or the continuing validity of the certification. The employer may ask for the same information on recertification as permitted for the original certification. The employer must allow the employee at least 15 calendar days to provide the recertification. Recertifications are at the employee’s expense.
 29 C.F.R. § 825.308.