Since most passports last for 10 years, that means a lot of people will need replacements this year and next year (2016 and 2017). State Department sources say they expect to issue more than 17 million new passports and renewals this year. That boils down to about 1.5 million more than in 2015. This means that renewals will take six weeks (or more) instead of the usual four weeks.
While new passports must be done in-person at your local post office, renewals can be done by mail. If you do so before it expires, that is ideal as there is a penalty if it has expired. You will need to provide two passport photos as well. Most Americans can renew passports by mail, for a fee of $110. http://travel.state.gov/content/travel/en.html
Source: New York Times