Whenever a candidate is projected to have 270 or more EVs.
That was simple enough to answer.
Yes and no. Technically, that is correct, but it is entirely possible for the networks to "drag things out" to avoid irritating people (or because the exit polls misfire, as they did in 2002).
Considering the networks are the ones that "project" electoral votes in the first place by "calling" states, I think my point stands that it is at that point, when a network has projected 270+ EVs, that that network will also declare the election.