Delivery times for Certified Mail can vary.
The USPS says it will take a minimum of 3 business days, but most letters should arrive within 5 - 6 days. We've seen some instances where it can even take up to 10 business days as well.
In the event that the postal carrier leaves a note for the recipient to come pick up the letter at the post office, it could possibly even take up to 20 days before the USPS returns it to the sender.
With all of that said, we say to allow 10 business days before you reach out to the USPS regarding the status of your certified letter(s).