The main elements you have to take into account when it comes to breakdown of any and all substances are: air, heat, light and moisture. If you place your plant matter inside a well-sealed container or bag with as near vacuum as possible, and put this in your freezer, you will effectively shield it from the elements at play. Using this technique it should last near infinitely. DMT is a rather stable compound, so other compounds may break down before infinity, but the described technique is the by far best method to store any and all materials.