Just as for the Big Earthquake that everyone is waiting for in the Los Angeles California (USA) region, we can only give probabilities to when such an event will happen. An article in Sky and Telescope magazine ( January, 1993), goes through some of the estimates for when the next deadly impact might happen.
Interplanetary rocks and asteroids come in a range of sizes. The most common are dust grains and pebble-sized particles. These slam into the Earth's atmosphere at 20 - 30 kilometers per second and are incinerated to make those beautiful meteor trails we see on many nights. Larger bodies are far less frequent, but we know they are out there. Every year, a small but dedicated team of astronomers discover new 'Earth-crossing' asteroids with sizes from a half mile across to a couple of miles. Every few years, one of these comes within a few million miles of the Earth. The extinction of the dinosaurs is blamed on just one such object colliding with the Earth.
Based on estimates of how common these large bodies are, and on the cratering record on the Earth over the last few 100 million years, astronomers estimate impacts with mile-sized asteroids every 50 - 100 million years. Smaller bodies are more common, so a 100 meter sized body would impact the Earth every 100,000 years..producing things like the famous Arizona Meteor Crater. Every 100 years, a body about 10 meters across strikes the Earth. The last recorded event like this may have been the Tunguska Event in 1909 in Siberia which flattened a forest for 20 miles from the impact site.
So, we seem to be overdue for another Arizona Meteor Crater event, and perhaps for another Tunguska event. About the later, on August 10, 1972 near the Grand Teton National Park in Montana, a large meteor entered the atmosphere and skipped right back out into space. In the August - October issue I recall Sky and Telescope magazine publishing a wonderful set of still photos taken by a vacationer in the Grand Tetons who just happened to have his 8-mm movie camera ready. These photos have been reproduced in just about every textbook on astronomy I have ever seen. If this meteor had struck the ground, it probably would have left a very big hole hundreds of meters across like a mini-Arizona Crater, so perhaps THAT was the one in 100,000 year event... or perhaps not!!
I wouldn't loose too much sleep over this, but I do think that we need to take these possibilities very seriously now that we are a global civilization. NASA should get very serious about keeping track of such space debris and try to find as many of these large bodies as possible. Just one of them striking the Earth could have devastating consequences.