I think they've been saying that the 50% mark for PGA Tour pros is a little under 8 feet.
http://www.pgatour.com/r/stats/filter/?1 has averages from different distances for the various players on tour. Looks like the average from inside 3 feet is something like 99+%. 4 feet, 90%. 5 feet and it's more like 80%. 8 feet, maybe a little over 50%. 10 feet, 40%. 15-20 feet, roughly 15-20%. (I'm mostly just looking at names on the list and picking the median. Obviously the percentages would vary based on whether you weight totals by number of putts or by each individual player, etc.)