Wiis by the end of the year: ~ 45-46M
Minimum Wiis sold in a month: 1.4M (as some of the January are flew in), 1.6M for the other months.
So by 1st April, it is certain.
Earliest possible?
Well, max Wiis in a month:
2.2M. but January will be a lot smaller, so little change of making it in February.
So sometimes in March. I'm guessing mid march.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS