Generally mimosa should not go bad even after months/years. If it looks fine and doesnt have some mutant fungus growing on it, its perfect There are reports of people using mimosa that was for many months or even years in a drawer without any significant problem (though as with anything, its always more prudent to store in dark, dry, cold place).

The main possible degradation is of dmt into dmt n-oxide. Dmt n-oxide is also psychoactive, but it is not soluble in naphtha, which may cause some people to get small yields when extracting from older bark. N-oxide is soluble in limonene/xylene/toluene/etc, so one could extract it with those solvents without loss of yield. Alternatively, N-oxide can be converted back to dmt using zinc dust, see link below

For oral usage of mimosa, formation of n-oxide wouldnt really affect much as it will be water soluble so one will still get the effects when consuming the brew.