There are many factors involved for keeping and storing a gingerbread house over long time periods: how well it's made, how well it's wrapped and under what conditions it is stored. Gingerbread's enemies are: accidental breaking, pesty pets, heat, humidity, water, mold, fungus, extreme changes in temperatures, bugs, and gnawing critters.
If your gingerbread project is made well (meaning using a recipe that makes it most sturdy rather than tasty), it will keep its shape better than one made with too much fats. Also, the fewer details it has that are frail (thin/easily broken), the better it will age over time.
Next, the wrapping. If you can wrap the project solidly in plastic, removing all air around it, it will preserve better over time (however, maybe your project's details won't hold up to this process). If you cannot remove all air, placing your project in a sealed plastic bag, removing as much air as possible, is the next best method.
Finally, storage. Make sure it's placed in a safe place, away from people/pet traffic. Heat, humidity, water, bugs, gnawing critters--all love to consumer sugary gingerbread. Therefore, even after encasing your project in plastic, if you can, also place it inside a plastic box with lid as well. Even if the lid clicks into place, air and critters can still get in. Therefore, further seal the lid by placing tape around it. Store your project in the coolest/dryest place you have that is not subject to extremes in temperature fluctuations. If you have an extra refrigerator or freezer, these are ideal locations in which to store projects.
Having said all this, I have made projects that have lasted years.
