Nate Cohn expects it to take a while. He writes that if "Obama ultimately wins the popular vote by a narrow margin, as suggested by the current average of national polls, Obama won’t lead the popular vote on Election Night and might not for weeks":
With the West Coast providing the margin of victory for any Democratic candidate in a close election, Republican presidential candidates outperform their eventual share of the popular vote until the West Coast reports its results. In 2008, California, Washington, and Oregon voted for Obama by a 4 million-vote margin, representing nearly half of his national popular vote victory.
But the time zones are not alone in delaying results from Washington, Oregon, and California. In most eastern states, the overwhelming majority of votes are counted by the end of Election Night, since only a small share of absentee or overseas ballots arrive after the election. But elections in Washington and Oregon are now conducted entirely by mail and 41 percent of California voters voted by mail in 2008. In some states, ballots only need to be postmarked by Election Day and it can take days before all of the votes arrive and weeks before they get counted, usually in modest batches once or twice a day.