David Cameron: Not sure exactly when, but it was several years before he became PM, so presumably 2006 or 2007.
Ed Miliband: 2010, after he became Labour leader.
Tony Blair: 2003, with the Irak War.
Gordon Brown: 2007, when he became Prime Minister.
Ken Livingstone: 2008, when he was defeated.
Nick Clegg: 2010, during the election campaign.
Boris Johnson: 2008, when he was elected.
Kevin Rudd: I think around 2008 or 2009, when he was already PM.
John Howard: Not sure, maybe after the 2010 elections by reading about Australian political history.
Julia Gillard: 2010, when she became PM.
Tony Abbott: 2010, during the campaign.
Stephen Harper: Probably 2009, when I read Hash's first rant against him.
Justin Trudeau: 2009 or 2010, when Hash educated me on Canadian politics.
Francois Hollande: Probably somewhere around 2004.
Nicolas Sarkozy: Probably around 2003, when he first made himself known as a fascist asshole.
Angela Merkel: 2005, during the election campaign
Vladimir Putin: Probably around 2003 as well.