It depends on the service that you are using to mail the letter. Certified mail typically travels along with the regular mail, so it will move at the same speed as the underlying service. This means that a certified letter sent through first class mail should arrive in 1 to 7 business days. Priority Mail will be faster at around 1 to 3 business days. The fastest option is Priority Mail Express. It should arrive in 1 to 2 business days, but it is significantly more expensive.