You have to provide the notice in time for it to be received by the depositary bank by 4 p.m. (its local time) on the second business day following the banking day on which the check was presented to you for payment.
For example, assume the check was included in a cash letter received by your bank on Friday, September 26, 2014. That's the day of presentment. Since business days are only non-holiday weekdays under Regulation CC, you'd need to ensure the notice of large item return is received (or retrievable) by 4 p.m. on Tuesday, September 30, 2014. The method used to send the notice is up to you, as long as it includes the information required by the regulation in section 229.33, and gets the notice where it needs to go by the deadline.