I'm with MOTORMAN37. Same as he, the shelf life of an ARF is probably longer than you would be interested in it, but I wouldn't put a time limit on it. The limiting factor would probably be invironmental conditions that would contribute to oxidation of metal parts in the hardware package or moisture that might cause the wood to form mold or rot, but that could be several (10, 20, 30?) years! Probably, the longer the model sits in the box the more the covering will wrinkle and loosen, but only up to a point. And I would imagine that in most (all?) cases, you should be able to re tighten the covering.

If you plan to store the model for more than a couple of years, you could consider lightly coating the metal parts with WD-40. If possible, I would store it indoors to keep it out of reach of mice or other rodents such as giant, ARF-eating rats! If your storage area is in a basement that may flood, store your model off the floor--an ARF I once had was sitting in about 1/2-inch of water when my basement flooded a couple of years ago. Luckly, all the parts were still in their individual plastic bags so nothing was damaged, except for the box.

If, as you stated, you want to store it for just a couple of years, I wouldn't worry about anything and just store it indoors if you can.

Tim