I got a flask for my b-day and i’m curious if there is a standard “use by” date on keeping liquor in the flask before you should replace it?
It should only remain in the flask a day or so.
You fill it up before heading out to “the game” or whatever, and it should be empty by the time you get home.
Especially if you’re filling it with a sweet liquor or liqueur.
Use it the same way you would use a Thermos for coffee or hot chocolate.
It’s NOT a long term storage device.
Why not? If the cap seals well it should be as good for long-term storage as anything else.
Of course, depending upon the circumstances you find yourself in whilst using it, you may ned to refill it quite often!
Never leave it in the flask more than a day or two, unless the flask is glass. Pewter flasks can give the liquor an “off” taste after a few days. I don’t know if that is from the tin, the antimony or what, but drink 'em empty and refill before the next game. 
Not just that pewter taste, but there may be a chance of lead in the construction - there shouldn’t be, but unless the maker certified it lead-free, there can be small amounts of lead in the pewter itself, or in the solder used to assemble it.
Fine for a day of tailgating, or a weekend at Faire, but don’t use it for long-term storage.
Until I drink it.
[sup]less than 24 hours, I guarantee ya![/sup]
Even stainless steel flasks advise to not store alcohol in them longer than a day.