Unfortunately, Snuff cannot be stored very long. As all tobacco products, the material gets dry and loses its taste. To keep your Snuff or Schmalzler fresh anyway, you should keep it where it is:
Some month of storage should be no problem. It depends on packaging and how long the Snuff was stored at the retailer, of course. Snuff should be aired once in a while to avoid a mouldy taste. This applies to Schmalzler also. If you are uncertain about the condition, just try it. If it feels too dry or develops a strange taste, throw it away.
Furthermore, you should only carry a one-day amount in your pocket, to prevent the snuff from losing moist caused by not tight snuff boxes and body heat.
John Arlott mentions the use of slightly salty water, "distributed by stirring, turning and, at need, sieving" to enliven older Snuff. (The Snuff Shop, John Arlott, p. 28) I have not tested this method yet, but it may be worth trying.