It took Arthur Conan Doyle three weeks to write A Study in Scarlet,
introducing Sherlock Holmes to the world.

It took Arthur Conan Doyle just three weeks to write the first Sherlock
Holmes novel, A Study in Scarlet in 1986. The novel first introduced the
classic literary characters of the detective Sherlock Holmes and his friend
Dr. John Watson, and is one of just four full-length novels featuring the
characters. Its title is in reference to a part of the story, in which he
refers to solving murders as “a study in scarlet.”