I'll give it til 2027. Just sayin'
I'm quite glad you mentioned that because, well by that time NOBODY will even care about the Golden Sun series anymore.
Like even if they pull another Dark Dawn 7 year thing and they release it 2016 or 2017, I think even around that time fans would have given up on the series and replaced whatever obsession they had with Golden Sun with games that are always around such as Pokémon or Zelda.
Especially considering how dull Dark Dawn was, if Camelot waits too long, I see the series being dead.
Which is why I'd like to see a 2014-2015 release, just finish the series, don't allow it to get to the point where nobody will even care whether or not Golden Sun 4 will be released.
By the way, yes I have been playing a lot of Pokémon and Zelda, I haven't stopped caring about the Golden Sun series yet, but I'm a die hard fan that considers Golden Sun better than stupid rpgs like Final Fantasy, Kingdom Hearts, Fire Emblem, and of course a fan Golden Sun fan like me that thinks that way is rather rare, most people in whom are Golden Sun fans aren't into it as much as I am, so in time they might lose interest, if Camelot takes too long to make Golden Sun 4.