The United States Postal Services reports that as of January 2011, it takes an average of one to three days for First-Class Mail to arrive at its destination. Priority Mail takes an average of one to two days to arrive, according to the USPS. USPS only includes the time a package stays in transit in their estimations. Packages dropped off at a post office during business hours usually leave that day. Packages dropped off after hours may take an additional day.