, but my fridge gets really cold. I'm sure that it depends on what's in between the layers.
I would imagine that if your cake is *not* cut, then it should last pretty darn long... I think I had one that was in there a month that was uncut, and it was still good (it may have been able to last longer). I think it's the sugar in the buttercream that kept any nasties from growing, sealing out air and all that jazz. Since it was a buttercream+jam fill, there wasn't a lot of filling that got sucked into the cake.
For some reason that I don't know well enough to explain, high amounts of sugar to low amounts of water somehow keep mold from growing. Honey, for instance, doesn't mold, and it's the same principle.
Illy
At least 30 days by industry standrds... so longer in actual time.
i forgot to say that it was a cupcake cake! does that make any difference? 
If your buttercream has crusted over, it should be fairly easy, and not stick... just take some pieces, lay them over the top, and tuck them underneath. Alternately, you could also take a large plastic bag and slide it, board and all, into the bag and close it up. You really just want to minimize the airflow around the cake itself, and not mummify it. 
Illy
how can i wrap it, if it's already frosted and decorated?
I place mine in a cake box and place that inside a plastic bag. Works every time and no damage due to the box!! 
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