USPS packages stay in transit for 1 day to 3 days depending on how far the package has to go.
In Transit means the package is traveling down the road in a semi truck or on an airplane to reach your destination.
Mail or packages can stay in Transit through USPS for 2 days or even 3 days before the tracking updates.
In Transit means the package is traveling down the road or on an airplane to reach the destination and so the package will not be scanned again until it reaches a sorting facility.
Once the package reaches a sorting facility the package has to be unloaded from the truck or airplane which has thousands of packages.
Then when your package gets off the truck and onto the sorting machine it is then sorted and scanned automatically and then your tracking information should update again.
When the USPS tracking says out for delivery today then the package should reach you that day.
When a USPS package is in Transit it means that the package is on a truck or plane being transferred to a different sorting facility.
That new sorting facility could be 20 miles away or it could be 100 miles to 1,000 miles away so it can vary as to the time it takes for the package to be in transit.
Some packages may stay in transit for one day and then some packages that are traveling farther may stay in transit for a few more days.
When the package is in transit the tracking will not update until the package reaches the sorting facility and then gets off the truck or plane to be handled and then scanned.
When it gets to your location then the tracking info for USPS should update to say it's arrived at your local post office.