It usually changes for me every few days. I'll mostly load up a song or two on youtube that has the right feeling for me for the page I'm on, then when I start the next page, I may switch that music up again. I do have some music that I occasionally load up on a playlist in iTunes, and let all of the music run it's course, but I've listened to those songs a million times.
Lately, I've been addicted to that Wreck It Ralph end credits song, "When Can I See You Again?", so I'd just find a 10 hour version and let it run it's course until I don't feel like listening to it. Before that, it was anime songs like Eureka Seven's Amazing Grace and Niji, Naruto Shippuuden's First opening and Blue Bird opening, and Durarara's Trust Me ending.
And now I'll be listening to that Superhero song that Savannah sent me on the theme song thread. It fits my comic really well.