500-600 miles.
You've got to get the mold release agent/lubricant to wear off. I've got a set of Conti Extreme DWs and after I had about 600 miles on them they handled very differently. I also needed to up the pressure about 3 or 5 lbs to stiffen the sidewalls to my liking.
http://www.tirerack.com/tires/tirete...e.jsp?techid=5