My Cowon player is rated at 50hrs but in my tests it was around 47hrs. This was not a continous test with music in which 50 was based on. I believe the 15hrs Sansa states is based on continous play as well. It should be fairly accurate but I am under the impression if you use the SD slot it hogs more battery (somebody correct me if i'm wrong here).

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne