My first tire change came at 6000mi, and wasn't really needed - I had at least 3k left in those D220s. My reason for change was the fact that the rubber was getting hard and brittle - due to the months of inactivity during the last half of my deployment to Iraq. D220s suck the fat donkey behind in terms of grip as well.
My switch was to a set of Michelin Pilot Powers, and I am quite pleased with them 3400mi later. They look to have at least twice that much remaining in them, and grip is stellar. I've ridden these tires superslap, twisty mountain road - and rain or shine, without any issues at all.