It actually took Hillary Clinton four days to concede the Democratic nomination after Barack Obama secured the necessary delegates in 2008.
As NBC News reported at the time:
Save to Favorites“Advisers indicated earlier Tuesday that the former first lady would publicly acknowledge in her speech that Obama had crossed the delegate threshold. But she changed her mind and refused to do so … Her advisers said they considered the delegate numbers to be unreliable, even as the AP estimated Obama had several more than the 2,118 needed to nominate.”