First of all, welcome to the measurement problem, which is one of the central interpretational issues of quantum mechanics.
There are not, as yet, quantitiative answers which can allow you to rule out the possibility that the entire Earth and various objects in low-Earth orbit are in a massive entangled state — if you believe that consciousness is a (poorly understood!) function of the brain as a physical system, that is. The reason why is related to the question of how you could ever hope to prove that the state that the Earth is in is an entangled one, rather than a decoherent mixture.
There are two hallmarks of an entangled state, as distinct from a mixed one.
One is that, if you have sufficient control over the system, an entangled state can be transformed into a pure one. Even in the case of the cat alone, let alone the Earth entire, the system is so massively complicated that you cannot hope to reverse the process which put its status as alive in doubt; to say nothing of the person who killed the cat (assuming that the cat was killed in the open rather than in an unpierceable box).
Another is that an entangled state is correlated not just with respect to one basis of measurement, but with respect to multiple bases of measurement — as in the EPR thought experiment, and which implicitly plays a role in protocols of quantum communication (such as quantum teleportation) which rely on highly entangled states. However, if the correlation is spread across very large numbers of particles (such as a system comprised of a cat, someone who may or may not kill them, and the rest of the Earth), the joint correlation would be hopeless to actually measure. This is a manifestation of the monogamy of entanglement, the principle that what distinguishes entanglement from 'classical' correlation is diluted when distributed across larger systems, and that maximal entanglement can only be manifested between a single particle with one other.
So while the entire Earth may in principle be in a massive entangled state — and indeed, an advocate of the Many Worlds Interpretation of quantum mechanics would say that of course it's in a massive entangled state; it always is, and so is the matter that makes up your body and your brain — there is no practical way that you could discover whether it is.
If you took the practical approach of saying that the state decoheres at the point that you could never hope to prove that it is entangled, then it is a question of technological limitations; and even if you could demonstrate entanglement in a system, there would be a question of the complexity of proving it, which I imagine would increase dramatically with the size of the system which you encapsulate into a box. So the proper question isn't really "when has the system decohered?", but "how quickly does the complexity of stopping decoherence grow?", where 'decoherence' is just the process of catching up more matter into the entangled state rather than the status of having lost entanglement. Concievably there would be a point where the resources of the entire universe would not suffice to distill the entanglement into a handful of particles where it could be observed, just as (and for precisely the same reason as) there would come a point where we lack the resources to reduce entropy in a sufficiently complicated system.
But in short, it seems to me that the problem is not a yes/no problem, but a how hard is it to do X problem. In that way, at least, we could hope to get a quantitative answer in the domain of our current theories.