USS Abraham Lincoln (CVN 72) just completed a nine month, $250 million refurbishment. This ship is the fifth of ten Nimitz class, nuclear powered, aircraft carriers in the U.S. fleet. It entered service in November 1989, and spends a lot of time in the yards.
There is a lot of maintenance involved with CVNs, enough to keep these carriers unavailable for over 20 percent of their career. Over its fifty years of service, each Nimitz class carrier has 17 planned trips back to the ship yard. There are twelve Planned Incremental Availability (or PIA) operations in which new gear is installed, worn or damaged stuff is replaced and any heavy duty work needed, is completed. Duration of a PIA varies with the amount of work to be done, but it can take several months, or a year or more.
Even more lengthily are the four Dry-docking Planned Incremental Availabilities (DPIA) operations, which are more extensive PIAs that include putting the ship into dry dock. These efforts can last a year or two. The one Refueling and Complex Overhaul (RCOH) is like the DPIA, except the ship is partially dismantled so that the spent nuclear fuel can be replaced. This takes a little longer than your usual DPIA, and often costs over half a billion dollars.