It takes 3 years to make another game, and they might spend 6 months making dlcs.... that puts us at July 2015. There will certainly be new consoles by then perfect timing for fallout 4. Skyrim is definitely their last 360/PS3 game.
I think it's closer to 5 years from start to finish, but they rotate their staff between projects. So concept work on Skyrim started as soon as Oblivion came out, but only a tiny team were working on the general themes and ideas for the game. Then as each team finished on Fallout 3, they got pulled off that and started on Skyrim, so in 2006 there were probably 3 or 4 people working on Skyrim but in 2011 it will have been over 100 - probably nearer 200 if you include testers. I'm guessing initial ideas for Fallout 4 would have started kicking around in, say, January 2009 if they're planning to release a game in 2013 (which wouldn't surprise me). I'd expect them to be thinking about Elder Scrolls VI already - just choosing a province and having a rough idea of what they want to achieve. (I may be wrong on those timescales - just trying to recall an interview I once read.)
Meanwhile, the media keep hinting about expecting an announcement any time soon about whatever will replace the xbox 360 - though with the Kinect it looks as though they're trying to extend the life of it, even though as we're seeing with a lot of games, the technology is struggling to keep up with what game developers are trying to produce. So 2013 is certainly not unreasonable to expect the xbox 720 (just making a name up here), or 2015 at the latest - which would make either Fallout 4 or Elder Scrolls VI the flagship title.
Just my personal guesses here, based on sheer speculation wot I read in 360 Gamer.