DMT can be used for years until it's nicelly stored.
As many things, a nice vaccum / sealed box in the freezer can keep it for years.
As many things, a long air / sun / T° exposition can alter it.
If you're ok with that, you can store it as an alcolic tincture (as other herbal tincture), some tek on the wiki explain how to dose accurately your tincture.
I stored some changa & freebase for month (maybe year) in a box at room T°, did not saw a deterioration in product & effect. The color can change with time.
Peace