An improved estimate of the number of nearby asteroids still capable of causing local destruction suggests these pesky rocks are likely to hit Earth about once every 1,000 years (Harris’ estimate is uncertain by a factor of about three.) Astronomers had thought such minor catastrophes occurred about once per century.

The new calculations, from Alan Harris of the Space Science Institute in Boulder, CO, show there are about 500,000 relatively small asteroids that inhabit roughly the same region of space through which Earth orbits. The asteroids are in the 50–75 meter (165–245 foot) size range.

Rocks this size can flatten a forest and would cause tremendous damage and even death if it hit a city.

From page 13 of this pdf
http://www.esa.int/gsp/NEO/doc/NEOMAP_report_June23_wCover.pdf

Table 1.1. The estimated frequency and effects of impactors as a function of size.

Impactor size (m) Mean impact Energy released
interval (yr) (megatons TNT)

30 200 2 — Fireball, shock-wave, minor damage.
50 2500 10 Tunguska explosion or small crater.
100 5000 80 Largest H-bomb detonation.
200 47,000 600 Destruction on national scale.
500 200,000 10,000 Destruction on European scale.
1000 600,000 80,000 Many millions dead, global effects.
5000 20 million 10 million Billions dead, global climate change.
10,000 100 million 80 million Extinction of human civilization.
Note: The energy release estimates assume a density of 3500 kg m-3 (stony body) and an impact velocity of 20 km s-1.

Being able to steer away those 200-500m rocks would be good and definitely the 1000-5000m and 5000m+ rocks.
Plus the intervals are statistical and if we perform a complete survey then we can get a better idea of actual dangers and timings.