USPS processing of packages takes around 1 day or less.
Once your packaged gets to a USPS sorting facility the USPS processes your package as fast as possible to get the package sorted and onto the correct truck or plane for delivery.
However if you order something online and the package is from out of state it can take 3 to 5 days to get the package because the package has to go through several more USPS sorting facilities.
The package goes through several sorting facilities and then gets sorted and loaded onto different trucks until it gets to the USPS sorting facility in your state that is closest to your local post office.
Then the USPS will update the tracking to say it arrived at your post office and is out for delivery.
The tracking for USPS packages usually updates every day around midnight when the package gets scanned again.