As of December 2008 I have only changed my back type one time. It's a slick att the moment and I need to do that.
I changed my fron one before trip over the alps 2008 - it's a slick now as well.
I changed my front tyre after 12500km and the rear tyre after 15500km.
Front tyre was worn down during heavy conering during my first alps trip and on the hot roads near Nice, the single most destructive driving that I did was on the road from Nice to Saint-Tropez.
View it on the map.
Road was so hot that the tyres melted, I burned my hand by touching them. After that I had to really just use the back break, but then that started smoking.