When you do something in earnest, you do it in a very serious and focused way. Once you actually start your homework in earnest, it won't take you long to finish it.
You can use in earnest to mean "earnestly," but also "more intensely than before." So if you first spend an hour goofing around, the moment you begin actively tossing clothes in your laundry hamper, sweeping up crumbs, and making your bed is when you start cleaning your room in earnest. An old-fashioned way to use this phrase is to mean "serious," in a sentence like "I looked at her and knew she was in earnest about adopting a rabbit."