About ten months. To be that big and not known yet ,has to be further away than Jupiter at present, and then you get the ten months from the time it would take to get to an Earth crossing position from beyond Jupiter, and add a bit for the possibility that it is behind the sun at first detection.

We have already found all the asteroids of ten kilometers or larger in short period Earth crossing orbits. We'd have spotted Jupiter family comets of that size by now also with aphelion same distance as Jupiter.

That just leaves the longer period comets with aphelion well beyond Jupiter. Those are the only ones left of this size that could hit us this century, and it's hard to say how many there are. One estimate is, 1% of the impactors of this size. But they are somewhat more devastating as they hit with much higher velocity. Going with that figure for now, it's perhaps a 1 in 100 million chance that we are hit by an asteroid this big in the current century. The hprobability anyway is low enough that they are not our priority at present. Main threat almost equally between one kilometer diameter asteroids and ones of order 100 meters diameter. The one kilometer ones are large enough to have global effects, the 100 meter ones are much more common but imipact on fewer people.

We should finish mapping out 99% of the one kilometer ones by the 2020s by which time the 100 meter ones would be the main ones to detect. It's a matter of priority - given limited funding we need to map the most risky ones first.

Bear in mind, never in human history do we have a confirmed example of anyone killed by an asteroid, though it may have happened with very small asteroids. Never mind hundreds, or thousands of people., These are very rare events. The ten kilometer or larger events were a 1 in a million chance for any century, but having searched and not found any in the NEO population headed our way this century, is now perhaps a 1 in 100 million chance.

See aso my http://www.science20.com/robert_...  though I need to update teh 1 in 10 million there to 1 in 100 million