Apple says 30GB 5G iPods have a maximum playing time of 14 hours. That figure is based on absolute continuous play (bear in mind stopping, changing playlists i.e. increasing disk use will use the battery more) with features like SoundCheck, the EQ and backlight off. 9 hours of battery time bearing in mind 'normal' everyday usage is fairly reasonable IMHO.
How long will your iPod last if you just let it play normal music files continuously without stopping/starting etc?