What a vial label does and does not say
A label carries a handful of facts and implies none of the rest. Knowing which is which prevents most of the confusion beginners run into.
Amount is not volume is not concentration
A vial marked 10 mg tells you the amount of substance inside. It does not tell you the volume of anything, because dry powder has no meaningful volume for this purpose. It does not tell you a concentration either, because there is no liquid yet. Concentration only exists after something is added, and it depends entirely on how much was added.
This trips people up when they see a number like 5 mg/mL somewhere and try to match it to their own vial. That figure describes a solution someone else made. It says nothing about yours.
Expiry and beyond-use are different dates
An expiry date applies to the product as manufactured and stored as intended — a sealed, unopened container. A beyond-use date is a different thing: it applies once a container has been opened or a preparation has been made, and it is generally much shorter.
There is no published, validated beyond-use dating for preparations made outside a controlled compounding environment. Not a longer figure, not a shorter one — the standards that produce those numbers take inputs that a home setting does not have. Anyone quoting you a confident number for this is extrapolating.