Probably not much, I can tell you that. In a perfect world I would recommend that the songwriter keep all of his rights in and to his songs. You have to understand what a publisher does before you can really make an informed decision about signing a publishing deal. A publisher can serve many purposes. Publishers can get songs placed in film, TV, advertisements and the like.
Publishers can assist in getting a recording deal for a songwriter that is also a performing artist.
Publishers can get a writer's songs recorded by recording artists. If you think you can do all of this and have a career, more power to you. I would recommend that you retain ownership of your own publishing rights. In most cases, however, the songwriter or artist, if successful, will not want to deal with this part of their business. Generally the songwriter wants to write songs, not collect money and plug those songs.