Delivery times for Certified Mail can vary. The USPS says it will take a minimum of 3 business days to be delivered, but most letters should arrive within 5 - 6 days. We've seen some instances where it can take up to 10 business days as well, but keep in mind that we don't control the delivery process of your mail as that's done by the USPS. The information contained in this article is either from them directly or from what we've observed over time.
Keep in mind that in the event that the postal carrier leaves a note for the recipient to 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).