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?