For the longest time I didn't believe that Episode Three was going to be much longer than Episode Two, but yeah, I can totally see it being a 10-13 chapter game (around the length of HL2) if it's coming in 2011.
Half-Life 2 - November 16, 2004
Half-Life 2: Episode One - June 1, 2006
Half-Life 2: Episode Two - October 10, 2007
Episode One supposedly began development shortly after Half-Life 2 shipped, so it had around 18 months of dev time. Episode Two began development around the same time as Episode One (iirc), so that had about 35 months of dev time. The release gap between Episode One and Two was about 16 months.
Doug Lombardi told us to expect a longer wait back in October 2008 (
http://archive.videogamesdaily.com/features/valve-doug-lombardi-left-4-dead-interview-p3.asp): "Yeah, the next time you play as Gordon will be longer than the distance between HL2 to Ep1, and Ep1 to Ep2." Given the bottom limit of 18 months or so and the development time of Episode Two, let's give an estimate of 48 months (or 4 years, still shorter than Half-Life 2's 6 year development time).
Assuming Episode Three began development at the release of Episode Two (likely earlier, but I'm being conservative), that would put the game somewhere in fall 2011.