Navigate to: Customers > Accounts > Manage Account > Additional Attributes
Additional account settings can be managed using account attributes.
Voucher Expiration Reminder Email (In Days) - Administrators can set the number of days prior to the voucher expiration date, a reminder email will be sent to the account customers to use the voucher.
This setting works for customers who are associated with the account. When there is no value saved against this field, then the value is taken from the Voucher Expiration Reminder Email (In Days) setting saved against the respective Store for sending the reminder email