Before I answer this, let it be known to all that I am REALLY EXCITED about Key as HC.
That being said, this question might not have an obvious answer right now. As has been said, recent history would say 5 wins as a floor and maybe 10-11 wins as a ceiling, with the possibility of a really special year every now and then.
What we don't know, and have no past history to guide us, is how exactly Key is going to build this team. We don't know how well his to be assembled staff will recruit. We don't even know what kind of offense we will be running yet. I don't think we will know for a couple of years how good he will or won't be, but I have a feeling we might be pleasantly surprised.