Hi Heather! It will stay fresh for many months since all of the ingredient are dry.
No mold or bacteria will grow within, so no need for a preservative.
However, the problem with making bath salts or soaks too far in advance is that they can easily clump up in their storage containers.
So you may go to give the gifts and they are just hard lumps.
Because of this trait, I make up bath soaks about a week or so before I want to give them to someone.