Since you mentioned Apple OS, that means you're using an iPod. Which model, though?
Also, please remember that Rockbox isn't 100% optimized for many of the PortalPlayer targets (iPods included), so battery life is not at its best. I have an iPod video that can give me nearly 9 hours of battery life, and an iPod color that only gives me 5 hours (the battery's dying on it anyway).