While we wait for results to start showing up, let’s lay out the rationale behind when we start talking about a particular race. Our traditional rule of thumb has been to wait until we have about 10% of results in, an amount that allows us to start drawing some conclusions—though of course, in closer races, much often remains up for grabs. In the past, we would rely on the percentage of precincts reporting to determine whether we'd hit this threshold, since 10% of precincts was a pretty close approximation of 10% of the vote.
But in more recent years, the increasing adoption of early voting and mail voting has rendered this metric useless, since "precincts reporting" often covers only ballots cast in person on Election Day. The Associated Press and other outlets have sought to adapt by coming up with an estimate of the total expected vote; the AP says their estimates are "informed by past turnout, advance votes cast and early returns," meaning they can shift somewhat over the course of a night.
We would still caution, though, that the AP estimate is an imprecise science, at best. A recent case in point is the special election in Texas’s 34th district, where for a long period it looked like there were a number of precincts outstanding until they finally decided that they were actually done. We will be on the lookout for situations like this that seem amiss, and encourage you to do so too.