Premise: Lamme Goedzak is a character in Charles De Coster's novel "The Legend of Thyl Ulenspiegel and Lamme Goedzak" (1867). He is the best friend of Thyl Ulenspiegel. While Ulenspiegel himself is derived from Dutch-German-Flemish folklore Lamme Goedzak is entirely created by De Coster. Despite this he has become one of the most recognizable Flemish folklore characters since.
Hypothesis: It took Charles De Coster a decade to finish writing "The Legend of Thyl Ulenspiegel and Lamme Goedzak"