I just read an article on how there's a 1 in 300 chance that a pretty decent-sized asteroid will strike the earth in the year 2880, or in approximately 765 years. One in three-hundred odds are pretty slim and I sincerely doubt the world is going to pay attention for very long, and we'll go back to looking for more probable threats.
My thought is this, though: Let's say that it is possible to determine the threat is very, very likely to occur. For the sake of argument, let's say that there's a 100% chance that asteroid will hit the earth and pretty much wipe out civilization as we know it, 750 years from now. What do we do then? Can we still "not worry" about it, at least for the time being? How long can we put it off? If it was a big report due at work, I know I could easily put it off for more than 749 years. I'd probably only need a solid week to put it together. But if the task entails getting a viable Earth human colony established on another planet, is 750 years even enough time?
It seems like a long time until you start to think about things like teraforming, or constructing large-enough settlements with everything they'll need for self-sustainability for thousands (or millions) of individuals. There may be a great deal of experimentation needed, of trial and error. If part of the plan is "let's just get to the singularity and then let the machines help" we might not end up being on the list of necessary items at all.
When I think of all of these contingencies and possible futures, my personal conclusion is that, if we found out tomorrow that we had 750 years to get off the planet in a survivable fashion, then we had better start in earnest RIGHT AWAY. What do you think?