The Airport Base has an IPv6 page that shows what the delegated /64 subnet is. All the devices on my network have autoconfigured themselves with addresses in that range. I appear to be able to manually set hosts to any address in that range as well and it seems to work fine. I do this on some of my server VMs because otherwise they generate a new random address every few days (privacy mode woo). The privacy addressing makes assigning a DNS name to said VM more difficult. I've had the same /64 delegated to me since June 2012.
I've heard that you can request a /60 prefix (16 subnets) if your router supports IPv6 Prefix Delegation (pretty sure the Apple one doesn't). I don't know exactly how you go about doing that though. I'd probably start with the HSI forums on dslreports.com