What size? 100 meter? Big apartment building?
That might only come with days notice -- but in days you can get far enough away. (And, they are beginning to catalog those in large numbers.)
A recent example of a very much tinier asteroid discovered before it arrived is noted in topics
Asteroid Impact Tonight!!! and
Near certain hit at Oct 07 0246 UTC!. It was so small, only a few meters, it only came with hours of warning. Still it was enough warning to provide escape, had it even been necessary. That's a cutting edge capability, and I expect it will be a good many years before everything so minuscule can be spotted in advance.
Larger, 1-kilometer-diameter asteroids are well on their way to being totally cataloged and predictable for decades out.
NASA JPL NEO Program Statistics
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NASA's search program designed to discover 90% of the NEO population (1 km in diameter or larger) within 10 years is under way.
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You can see for large objects the discovery rate is diminishing. It's getting harder to find new ones. Those bigger ones will come with warnings from months to years, again time to get away -- except for the extraordinarily rare, extremely large, so-large-we'd-already-know-about-it asteroid for which there was no safe distance on Earth, that would require some deflection, many, many decades or centuries from now, when we'll be able to do so easily.
Other rocks are out there: comets. Coming from farther reaches, they are harder to catalog in advance. Rarer, too.
So, as usual, why don't you worry about something that can really give you a bad day, such as the much more common: bad diet, not wearing seat belts, not getting enough exercise? Stop dying over and over in your fantasies. Worry about the important stuff. Please.
NASA JPL Near Earth Object (NEO) Program has stuff to read:
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Introduction & Overview
NEO Groups
Near-Earth Objects And Life On Earth
Target Earth
Near-Earth Objects As Future Resources
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Target Earth
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With an average interval of about 100 years, rocky or iron asteroids larger than about 50 meters would be expected to reach the Earth's surface and cause local disasters or produce the tidal waves that can inundate low lying coastal areas. On an average of every few hundred thousand years or so, asteroids larger than a kilometer could cause global disasters.
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Ahem:
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No one should be overly concerned about an Earth impact of an asteroid or comet. The threat to any one person from auto accidents, disease, other natural disasters and a variety of other problems is much higher than the threat from NEOs. Over long periods of time, however, the chances of the Earth being impacted are not negligible so that some form of NEO insurance is warranted. At the moment, our best insurance rests with the NEO scientists and their efforts to first find these objects and then track their motions into the future. We need to first find them, then keep an eye on them.
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