Most gymnasiums are for public use and get a heavy dose of daily traffic. Manufacturers and installers of sports floors recommend that they be screened and recoated once a year to help the surface perform like it should. The process of screening and coating is the same as it is during installation. A single pass is made with a circular sander using a fine screen, and one layer of urethane is applied to the top. Think of each pass with the sander and urethane topper as a brand new top layer to your floor. The same thing applies to your home hardwood floor, although it's not necessary to do so once per year.
A gym floor takes a couple of days to screen and recoat and it's typically ready for basketball after about 72 hours of curing. If the gymnasium is commonly used for other nonathletic purposes it may need more than one screen and recoat per year. One example is a high school gym that's also used for assemblies and school dances. Chairs, tables and non-sneaker shoes can wear out a gym floor much quicker than if it's only used for sports and recreation.