how long does a fondant decorated cupcake lasts?

I have some in my Bakery that have been on display for a year...they just dry out and don't go moldy.

If you mean to eat it, I would say as long as a frosted cupcake, maybe a day more because if your fondant goes right to the edge of the cupcake the air won't get to the cake part as easily, and assuming you have a good layer of buttercream under the fondant helping to keep the cake moist. Cupcakes have smaller amounts of cake that a whole fondant covered cake, so it won't last as long as a cake if you intend to eat it. Although it will also depend on where and how you store it. 

ABut how should i store it? Does it need a speacial treatment?

AIn a cardboard box or loosely covered. Airtight containers will make fondant sweat and droop.