As an expert sommelier and brewer, I have had experience with storing alcohol in flasks. When it comes to the length of time you can leave alcohol in a flask, there are a few factors to consider. The material of the flask, such as stainless steel or glass, can affect the freshness and taste of the alcohol over time.
For stainless steel flasks, it is generally recommended to limit the storage time to three or four days. This is because stainless steel is not completely airtight, and over time, the alcohol can oxidize and lose some of its flavors. Additionally, the metal can impart a slight metallic taste to the alcohol, especially if it is stored for an extended period.
On the other hand, if you're interested in storing alcohol in a flask for a longer period of time, consider using a glass flask instead. Glass is more airtight and does not react with the alcohol, preserving its flavors and freshness for a longer duration. Glass flasks are also a popular choice for long-term storage of spirits, such as whiskey or brandy, as they do not alter the taste of the alcohol.
In my personal experience, I have found that storing alcohol in a stainless steel flask for more than a week can result in a noticeable deterioration in flavor. I once left a flask of vodka in a stainless steel flask for two weeks, and when I finally opened it, the vodka had a slightly off taste and lacked the crispness it originally had. Since then, I have made it a point to finish off or change out the contents of my flask within a few days, or a week at the most, to ensure maximum freshness.
To summarize, stainless steel flasks are best for short-term storage of alcohol, ideally limited to three or four days. For longer storage, consider using a glass flask to maintain the freshness and taste of the alcohol. Ultimately, the choice depends on your personal preference and how long you intend to store the alcohol.