Stable bonds, as hormones, cannot degrade of their own volition (outside catalysts are required to alter them). What can, however, go "off" is the suspension oil. And what prompts that degradation, is excessive exposure to heat, light (whether direct or indirect), and oxygen. So, when's the last time you had to toss out cooking oil? It's more or less the same as what's suspending your Testosterone... it very rarely goes off. It takes years... and that's with quite a bit more exposure to the aforementioned than does your T vial.
What would happen if your suspension oil somehow mostly broke down, is your levels would rise exponentially for a day or so (and depending on volume, potentially to adverse effect), and then plummet. Because it's the oil that circumvents your body from metabolizing the entire dose at once, instead prolonging the release in metabolite, for several days to a fortnight (on average; everyone's metabolic rate, therefore absorption rate of oil-suspended medications, is different).
A 10mL vial lasts me over a year, for nearly the past two now. I have noticed no degradation, nor do my labs reveal any either. But then, I do keep my vials in a cool, dark, place. In a pouch, still in it box, also - for good measure. I have another already beyond the 'use by', and I'm certain it will be just as 'good' as were it fresh by the time I get to it in a few months. Even if not, the 'degradation' won't be enough to discard it.
The reason most medications, and most edibles for that matter, have 'use by' dates is for purposes of legalese. Not because the substance will actually 'expire', or be 'harmful', if used beyond that date.
In short, this is not a concern. Store your vials appropriately and you need not worry about them.
