Five years pass between How to Train Your Dragon and How to Train Your Dragon 2.
The sequel begins five years after the events of the first film. During that period, Hiccup and Toothless explore the surrounding lands while Berk develops a peaceful relationship with dragons. The time jump allows the sequel to show the young characters as older and more experienced riders.
This interval is often confused with Hiccup’s age. Hiccup is 15 in the first film and 20 when the sequel begins, which is why the five-year gap is especially noticeable. The question asks about the chronology between films, not simply the character’s age.
The time jump also explains several changes in Berk. Dragons now live openly among the Vikings, the riders use improved equipment, and Hiccup has become more confident as a potential leader. The sequel’s larger political and family story depends on this period of relative peace before Drago Bludvist threatens the region.