I continually add to my playlist. Currently it sits just above ten hours. I had one around 38 hours, but I got tired of some of the stuff, instead of weeding out what I didn't want to hear, I just made a new one with only what I really wanted to listen to.
Yes, there's dubstep in my playlist; don't hate.
I have some excellent tracks from OCReMix, a bunch of stuff from newgrounds.(mostly dubstep, house, trance, and techno), some Ratatat, Buckethead, Pretty lights, deadmau5, a few Skrillex tracks, Dominic Gaudious (Guitar + Didgeridoo FTW), a few songs from Muse, and a couple from DragonForce to round it all out.
I also have a decent collection of music from some of my old favorite games. Ridge Racer 4, Need for Speed 3 and High Stakes, Timesplitters 1, 2, and Future Perfect, and Guilty Gear.
That covers most of it for now, I've got a lot more but either don't feel like listening to it or haven't bothered to sort it all into a proper playlist. (For instance, I have every track on OCReMix, about 700 more from Newgrounds, and 24,000+ MIDI tunes which comes to well over three straight weeks by itself.