The earliest is mid/late 2011 I think, because Nintendo will not launch the DS' successor one year after DSi XL. They will at least pause long enough to see the reaction to that update.
Beyond that, it will depend on software. And I mean 5m and 10m unit selling software, which can keep hardware sales high. The tons and tons of 1-5m unit selling software I don't think can keep sales at 25m a year without some big 10m sellers too.
2010 will see Pokemon HG/SS sell 10 million units. Nintendo may push Tomodachi Collection as a major title in the west, especially Europe. But who can tell if it is another Brain Training. Beyond that, what is there?
If Nintendo comes up with some more ridonkulous mega-hits, it can do like Game Boy, which got Pokemon so late in the cycle, and extend its life as the flagship system beyond a decade. Without those, holiday 2011 or 2012 seems more likely.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
