How much notice would we get if an asteroid the size of the one that destroyed the dinosaurs was headed our wa?

We would see it years in advance. Possibly up to 15 years in advance with modern technology. Most asteroids travel about 50,000 miles per hour, which is still somewhat sluggish if it is coming from the asteroid belt. I'd say a bigger worry is if a comet was headed our way, because we'd have much less time to prepare considering they travel faster than asteroids and hook around the sun without us seeing it.