I live in an area with a lot of potholes and commute daily (between 10-20 miles depending) on a modern road bike. Today while coming in hard (18+mph) to a stop, I went over a worn paving layer with the front heavily loaded. I heard a loud CLACK, and my first thought was to look to the carbon fork. No cracks, but it got my mind on to my head set and head tube. (It was probably just the rear derailleur swinging out and coming down on the B screw.)
What's the minimum mileage interval for headset tightening, and what are the symptoms that indicate it's time? (It's not covered in the Optimal Maintenance question.)