Given that there are literally thousands of NEAs (Near Earth Asteroids) roaming within a few million miles of Earth, it's only a matter of time until another one crashes into our world.
Some of theses rocks in space have huge elliptical orbits around the Earth that take years to complete, and some pass by only once and continue into the unknown. Since these objects are generally less than 1km (about 3600feet) in diameter, they are quite dim as they don't reflect much sunlight. This makes them tougher to find with a telescope.
Furthermore, many are covered with a dark dust further limiting their reflective properties.
In March of 2009 a sizable meteorite, about 100 feet across, was discovered a mere three days before passing near to Earth. It flew 45,000 miles from the Earth's surface. This is about twice the altitude of orbiting weather satellites. This was astronomically speaking a VERY close call. Had it hit in an urban area, it could have destroyed an area the size of a medium-sized city and killed hundreds of thousands.
As more and more of the sky gets scanned by both Earth-bound and orbiting telescopes, the overall risk of a surprise should decrease over time. The problem is twofold: the effort required to cover the entire sky is a monumental task, and the ability to see these dark ghosts of space in the first place.
The Meteor Crater in Arizona, a 50,000 year old example of a large impact, is the most well preserved example because of the dry climate. Many others craters of younger age (like Tunguska in 1908) were buried or eroded by more active weather patterns.
It's likely many ancient episodes of this type were dismissed as the work of various gods and not well documented: for instance the Roman god "Vulcan" is the origin for the word "volcano". Apollo was supposed to have carried the sun across the sky each day. So a rock falling from space (fireball) was probably a bad omen requiring prayers and sacrifice and not much else.
We know from geology they have been happening for a very long time. A much lower Earth population in the past also reduced the odds of detecting each event. Of course many have probably harmlessly fallen into the oceans, never to be seen and escaping history.
All of this to suggest that the next rocky visitor from above may come with some warning, if we are lucky enough to see it; but just as likely it will arrive uninvited and unannounced.
Thus, in reality the answer should be "it's anyone's guess".