Everyone handles this differently, though. :)
The gist is this:
Synthetic wigs will keep indefinitely if not worn. I definitely suggest taking wigs out of storage every couple of months to wear them or place them on a mannequin head. If you leave them in the box indefinitely, they can become misshapen.
Human hair wigs will need to be moisturized periodically, even if you aren’t using them. Just like leather, if you let them go too long without some moisture, they will become dry and brittle and begin to break down. As long as you keep this in mind, though, you can store them for many years without any problems.