So, if I understand the chart correctly, this means that the average Wii is being played ~ 8 hours a month (~ 500 minutes).
Hardly a lot. In fact, it is around 15 minutes a day in average.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS