Analysis of leafs such as chacruna have NOT shown any N-oxide even when specifically looking for it using reference standards.
I'd think that they can be stored basically indefinitely, at least in terms of DMT remaining stable.. If they are improperly stored, maybe they get moldy or something, and I don't recommend drinking a mold brew, though you could probably extract and purify DMT safely from it even in that case.
So just in case, better store in cool/dark/dry, as with everything.