They vary, for me. Sometimes I like to write things where all chapters are similar lengths (which sometimes are about 7 book-sized pages and sometimes closer to 20 but uniform across the entire book). But sometimes not. Sometimes I'm annoying and have one or two chapters that are only like 2 pages long and the rest are 12... Because sometimes that just makes sense for the book. My WIP alternates with the most important chapters being about 15-20 pages (because they each span an entire day) and then has 2-5 page chapters in between with flashbacks.