There are a few different light novels with very long titles, but the longest one that comes to mind is 八男の母、八女の母、八男と八女の父、八男と八女の八人の兄弟の姉妹の長男の妻、そして八男と八女の八人の兄弟の姉妹の長女の妻 (The Mothers of the Eight Boys, the Mothers of the Eight Girls, the Father of the Eight Boys and the Father of the Eight Girls, the Eldest Brother's Wife of the Eight Boys and the Eldest Brother's Wife of the Eight Girls, and the Eldest Daughter's Husband of the Eight Boys and the Eldest Daughter's Husband of the Eight Girls) by Munesuke Natsuji.
If you're interested in light novels with long titles, you might also enjoy 千年王国物語 (The Tale of the Millennium Kingdom) by Doppo Kunikida and 七つの大罪 悪魔の逆鱗に触れた男たちの歌 (The Seven Deadly Sins - The Song of the Men Who Touched the Devil's Reverse Scale) by Shuka Matsuda.
