In my opinion, we can't stop supporting now. But, may be by the end of this year.
Look at the following stats :
Month by month stats of popular browsers : http://www.w3schools.com/browsers/browsers_stats.asp
Month by month stats of Internet Explorer : http://www.w3schools.com/browsers/browsers_explorer.asp
Clearly, popularity of IE6 is steadily decreasing, but is still significant enough.