In 1984, what do Julia and Winston decide about the longevity of their relationship?
Both do agree on one thing--they discuss what it would mean if they are captured by the Thought Police and decide that betrayal for them occurs only if they deny their love for one another. Winston and Julia believe that if the Party captures them and they confess to numerous "offenses" that that means nothing as long as they do not betray their love for one another. Of course, O'Brien knows this and is eventually able to get them to choose themselves over their lover. While in Room 101, Winston finally cries out, "Do it to Julia!" so that he will not face the rats himself. This is the ultimate betrayal of love since it is supposed to be represented in one's willingness to sacrifice his or her own good for another's sake.
So, while Winston might not have thought that he and Julia would have any future together, Orwell does present him as believing that he would not betray his love for her.
What they decide about this shows the differences in their basic outlooks. Winston is a total pessimist while Julia is an optimist.
Winston believes that their relationship is doomed and so are they, personally. He says that they are "the dead."
By contrast, Julia feels a lot more positive. She does not think that they are doomed. She thinks that they really do have a chance to continue to have a relationship for a relatively long time.
As it turns out, of course, Winston is right and Julia is wrong. They do not have much time together at all.