After first meeting Dill in the first chapter, Dill leaves in the beginning of Chapter 2. As soon as school starts, Dill has to return home to Meridian. Then after the scene in chapter 6 where Jem lost his pants in Radley's fence, Dill left again for home. Chapter 14 is where he comes back and hides under Scout's bed (he had run away). He appears in most of the chapters from that point on until chapter 25. In chapter 26, Scout is in 3rd grade, and Dill always goes home when school begins. It didn't always say when we would leave, but it was always when school began. Using the following link, choose the coordinating chapters to back up this info.
dill leaves in the beginning of chapter 2.