The longest name that I can find (excluding spin-offs, movies, second season, and special editions etc, that have extended subtitles as opposed to the main show) is "Shuumatsu Nani Shitemasu ka? Isogashii Desu ka? Sukutte Moratte Ii Desu ka?" which has 75 characters (61 alphanumeric) and translates to "WorldEnd: What are you doing at the end of the world? Are you busy? Will you save us?"