Concerning Lennie's dream in Of Mice and Men, deciding exactly at what point it's destroyed is a little like splitting hairs, as they say. Realistically, Lennie and George never had a chance of fulfilling that dream, anyway, so it was destroyed long before Lennie kills Curly's wife or before George kills him.
On the other hand, Lennie's dream is an illusion within his mind, so technically, it would not be destroyed until Lennie is destroyed, since Lennie is not enough aware of the consequences of his actions to understand what's coming.
George's part in the dream was equally an illusion and virtually impossible to fulfill. But certainly, for George, if he really believed in the dream, his belief would have died when Lennie kills Curly's wife. He understands what this incident means more than Lennie does.
Lennie's dream may continue until Lennie's death, then, but George's dream certainly dies with Curly's wife.
In my opinion, the dream that Lennie and George had was dead as soon as Lennie killed Curley's wife. I think that at that point, there was no way that they were going to get away and be left alone.
When Lennie had done bad stuff before, like with the girl up in Weed, it had been relatively minor. The two men could always just move on and keep dreaming. But now, he's actually killed a person -- and a relatively important person at that since her husband's dad was the ranch owner.
Surely the police would never have just let them get away. So the dream was dead at that point.