On the main screen of your DS click on your activity log. It looks like a little bar graph. Go to Software Library and then click on your New Leaf Symbol. It will tell you how many hours you have played.
Over 1700 hours when I looked last month. Not sure if that info transfers over so haven't checked my activity log recently. 1700 hours well spent imo though ^_^
Oh, I play for 9-10 hours a day straight and I leave my 3ds open with animal corssing on when I'm on tbt and other web sites, so I guess there's a thing for you!