Not exactly sure why you need to charge the battery for 8 hours the first time, but like most rechargeable batteries I'm assuming it's for breaking in the battery and helps the battery last it's full lifespan. The batteries already come with a charge though, just not sure if it's a full charge or not. I read in a thread somewhere JD from truesmoker.com was telling people it was ok to just start using one of the batteries while the other one charges so you can enjoy using the dragon right away. I don't think he said how that would affect the battery, but did say he would still guarantee warranty if anything happened.
I couldn't wait, so I did just that when I got my dragon. Used one right away while the other charged. I didn't pay attention to when the light changed on the charger, but it does start to alternate between red and green. Not sure what that means, just assuming it's charged already especially if it's been a few hours. For battery break-in though, you're supposed to leave it charging for the full 8 hours, even if the light turns green. Both batteries seem to be working fine so far, but I've only had it for a week. Hope that helps.