"Heavy DRM not only slows down an MP3 player but also sucks the very life out of them." So says this CNET mp3.com article on realistic battery life.

Here's more:

Those who belong to subscription services such as Napster or Rhapsody have it worse. Music rented from these services arrive in the WMA DRM 10 format, and it takes extra processing power to ensure that the licenses making the tracks work are still valid and match up to the device itself. Heavy DRM not only slows down an MP3 player but also sucks the very life out of them. Take, for instance, the critically acclaimed Creative Zen Vision:M, with a rated battery life of up to 14 hours for audio and 4 hours for video. CNET tested it at nearly 16 hours, with MP3s--impressive indeed. Upon playing back only WMA subscription tracks, the Vision:M scored at just more than 12 hours. That's a loss of almost 4 hours, and you haven't even turned the backlight on yet.

We found similar discrepancies with other PlaysForSure players. The Archos Gmini 402 Camcorder maxed out at 11 hours, but with DRM tracks, it played for less than 9 hours. The iRiver U10, with an astounding life of about 32 hours, came in at about 27 hours playing subscription tracks. Even the iPod, playing back only FairPlay AAC tracks, underperformed MP3s by about 8 percent. What I'm saying is that while battery life may not be a critical issue today, as it was when one of the original hard drive players--the Creative NOMAD Jukebox--lasted a pathetic 4 hours running on four AA nickel-metal-hydride rechargeables (and much worse on alkalines), the industry needs to include battery specs for DRM audio tracks or the tracks we're buying or subscribing. Yet, here's another reason why we should still be ripping our music in MP3: better battery life, the most obvious reason being universal device compatibility.