Tonight, right after results from South Dakota and Montana come in, hopefully not long after the evening news cycles; or very quickly in the next day or so.
In "reserving the right" to challenge the Credentials Committee's ruling this weekend, and setting Harold Ickes up as the angry Clinton partisan, Hillary's concession will appear particularly gracious according to the frame her campaign has already orchestrated.
And she will "win" while still relatively ahead. Before scores of superdelegates jump onto the Obama bandwagon tonight and tomorrow morning, she needs to end her candidacy before the electoral math finally places her chances at zero. In threatening to declare victory before Clinton formally concedes, the Obama forces are also trying to force her hand. She can't concede if she has unambiguously lost.
Now she'll still have some leverage, if she concedes today. 17 million voters selected her: so in or out of the race, she will have a voice leading up to the Democratic convention, in the selection (and possibly occupancy) of the VP, and the fall campaign.
And if I were her, I would "invite" the remaining superdelegates to endorse Obama in her concession speech. This would be the hardest thing probably that she'd be doing in her political life, but she should take advantage of this final opportunity for a gracious exit from the stage. (Though I will say that the longer she waits, the more likely she's holding out to get the VP position.) And thus she will begin the resuscitation of the tarnished Clinton brand, and the healing of the party.