My guess is 40 years after the first ice free Summer (with a range of 30 - 50 years). (neven1.typepad.com)
This is also apparent in Wang & Overland's 2009 study «A sea ice free summer Arctic within 30 years?» (realclimate.org)
Instead, a rather casual article in the Independent showed the latest thickness data and that quoted Mark Serreze as saying that the area around the North Pole had 50/50 odds of being completely ice free this summer, has taken off across the media. (realclimate.org)