How long will Nintendo store my dream town?

According to the the Animal Crossing Wikia:

If the dream world is not updated or visited for a very long time, it will be automatically deleted from Nintendo's servers. This will wipe every collected data of the dream world including visit counts.

So I'm wondering, how long can my dream town go without being visited or updated before Nintendo's servers delete it?

It is not currently known how long Nintendo will keep your dream town. But if the dream world is not updated or visited for a very long time, it will be automatically deleted from Nintendo's servers.

http://animalcrossing.wikia.com/wiki/Dream_Suite