Manager roles, responsibilities, compensations are based on the agreements that are established. You need to put some thought into what you're wanting them to do. Get someone that's enthusiastic about your music, for sure.
For a lot of indie musicians, you hire a manager when it's worth paying for one and/or when you can no longer handle all the responsibilities that the manager can do. So before musicians are ready or able to hire others to help with their musical career they look at putting together a plan and setting timelines for themselves. Much of the Marketing and Promotion to be considered, are dependent on what you want and what effort you can put into the more business side of music. Items that you could consider are those such as artist bios and images, social networking accounts, website, demos or music available to public in stores, streaming, YouTube, Soundcloud, etc.
We have a Marketing and Promotion Club here on the boards. Join us if you're interested in learning or doing some of the promo tasks that you are wanting a manager to do. We also have Marketing and Promo challenges that can get you thinking.
Peggy