As mentioned above, it should take over a year, and then it is pretty hard to tell how long it is from that. Could be 10 months, could be 20.
I just hope they don't have insanely high specs :( (ala higher than Cryisis is now, which is very likely).
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS