Could Snape have delayed fulfilling his Unbreakable Vow?

Question

When Snape made the Unbreakable Vow with Narcissa Malfoy, was there a time limit attached to it? Here is the dialogue from it:

Narcissa: "Will you, Severus, watch over my son, Draco, as he attempts to fulfill the Dark Lord's wishes?"

Snape: "I will."

Narcissa: "And will you, to the best of your ability, protect him from harm?"

Snape: "I will."

Narcissa: "And should it prove necessary... if it seems Draco will fail... will you carry out the deed that the Dark Lord has ordered Draco to perform?"

Snape: "I will."

The first two parts of the vow are ongoing: if Snape fails to watch over Draco or if he fails to protect him from harm, then Snape broke the Unbreakable Vow and will die.

But what about the third part, requiring Snape to kill Dumbledore? Could Snape have just said: "Wait! I'm still planning on killing him! Well, I'm planning on doing it sometime…eventually…maybe in a few years…but the point is, I haven't broken the Vow yet!"

Obviously, Snape did fulfill the vow (there were a lot of advantages to have done so), but could he have just delayed the last part of the deal indefinitely? Could he have chosen not to kill Dumbledore when he had the opportunity and still get around the Unbreakable Vow since he would have had many more opportunities in the future?

Answer 1

The vow is made in the context of Voldemort having tasked Draco will killing Dumbledore as punishment for Lucien's failure. In the chapter Spinner's End we find:

“Then I am right, he has chosen Draco in revenge!” choked Narcissa. “He does not mean him to succeed, he wants him to be killed trying!”

When Snape said nothing, Narcissa seemed to lose what little self-restraint she still possessed. Standing up, she staggered to Snape and seized the front of his robes. Her face close to his, her tears falling onto his chest, she gasped, “You could do it. You could do it instead of Draco, Severus. You would succeed, of course you would, and he would reward you beyond all of us —”

I can't find anywhere in the text where it states Voldemort will kill Draco if he fails, but that is certainly the implication.

So the unbreakable vow is not just to kill Dumbledore at some indeterminate point in the future. It is to save Draco's life by killing Dumbledore if Draco fails. That means Snape would be breaking the vow if he didn't kill Dumbledore immediately after Draco's attempt/opportunity.

Later:

As DavidS points out in a comment, the third vow is:

And, should it prove necessary … if it seems Draco will fail …” whispered Narcissa (Snape’s hand twitched within hers, but he did not draw away), “will you carry out the deed that the Dark Lord has ordered Draco to perform?"

(emphasis mine)