Just as a heads up, but on other players Rockbox gets MORE battery life than the original firmware. While we haven't done this in a color screen player yet, it's possible. At the very least we should eventually reach a "similar" level.
The main problem with battery life is that it's not a focus right now. Rockbox on iPod is still in fairly early development, and since it's primarily for developers, and people looking for bugs, more time is spent fixing major bugs than optimizing code (which is what will improve battery life.) Once it gets closer to being ready to release, and there are less bugs about, more time will go into optimizations and similar things.
Don't worry, the intent is to have decent battery life on it, in the end. It's just early.