It depends which hormones, and in what conditions. Very simplistically, complex chemicals like the gibberellins will be less stable than relatively simple ones like IAA. Ones with un-conjugated double bonds like ethylene will be less stable than ones with saturated bonds or aromatic ring systems. All will be less stable in the presence of light, especially UV light, than in the dark, all less stable in water than as dry powders. And of course many will be actively metabolised by plants themselves, so they will be far less stable in living plant tissue than in dead tissue. Beyond those sorts of generalities, I do not think you can say - it will vary.
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