Not sure of an exact time but i have found multi-year old (3+) dmt freebase in the past with no noticable loss in quality it just develops a yellow tint the older it gets
Generaly storing anything in cold temps will slow chemical reactions so if you were to store freebase at -80ºC id be willing to bet that it would last longer than any of our lifetimes
Salts last a fair bit longer than freebases
Sorry i cant give the exact number of years it takes for dmt to degrade by 50% or anything like that but unless you expect to save it for ten years or so i wouldnt worry too much about it
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