When will Superman become public domain? Superman was created back in 1933, so he will turn 100 in just 19 more years.
Not sure how it works, don't they go by day of first publication? If so
Superman 1938, he has to get in line behind (just to name a few favorites

) :
1887 - Sherlock Holmes
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1897 - Dracula
1897 - The Invisible Man
1898 - The War of the Worlds
1898 - The Black Corsair
1900 - Dorothy Gale, Wizard of Oz
1903 - The Scarlet Pimpernel
1904 - Rima Green Mansions
1909 - The Phantom of the Opera
1911 - Fantômas
1912 - Tarzan
1912 - John Carter
1915 - Doctor Syn
1916 - Judex
1919 - Zorro
1926 - The Bat
1928 - Solomon Kane
1928 - Mickey Mouse
1929 - Buck Rogers
1929 - Popeye
1930 - The Shadow
1930 - The Bat Whispers
1931 - Dick Tracy
1932 - Conan
1933 - The Lone Ranger
1933 - Doc Savage
1933 - The Spider
1934 - Flash Gordon
1934 - Mandrake the Magician
1935 - Doctor Occult also Siegel & Shuster
1936 - The Green Hornet
1936 - The Phantom
1936 - Ka-Zar
1937 - Prince Valiant
1937 - The Hobbit, Gandalf
1937 - Sheena
1938 - Superman Action Comics #1
Although many of those seem to have lapsed, maybe they weren't defended? Or it depends how powerful their lawyers were

.
The likes of Mickey Mouse and Superman lapsing will probably force laws changed, if they haven't already.