According to the gentleman hip flask guide it says:
Storage
How long may you keep the spirit in a flask? You should not leave it for longer than a week, as an upper limit; ideally, you should carry your day’s quota and drink it on the same day. Some say that anything over three days will make the beverage acquire a metallic taste from the flask. If you do not plan to drink the flask content promptly, transfer it to a glass container. Also, put the flask with alcohol in a cool and dark place; this may slow the development of the metallic taste.