Short-term: hold the freshness
Short-term storage for red tea means keeping it tasting like itself for as long as possible. Red tea is fully oxidized, so there is nothing left to transform. What you are protecting is aroma, and aroma leaks.
The environment is cool, dry, and dark. Around 20 degrees, below 50 percent humidity, in a cupboard that does not sit near the kettle or the oven.
The container needs a genuine seal. Red tea is rolled rather than pressed, so it has a lot of exposed surface, and it gives up its aroma to any container that is not airtight within weeks rather than months.
The timeline is six months to two years. Inside that window a well-stored red tea stays pleasant, and the most likely failure is that it simply becomes less interesting.
Long-term: change the goal
Long-term storage for red tea is mostly a mistake, and it is worth saying plainly. Red tea does not improve with age. It holds, and then it thins out.
There are two narrow exceptions. Traditional smoked Lapsang Souchong, dried over pinewood fire, keeps its character for several years and some people prefer it with a little age on it.
The other is sun-dried Yunnan red tea, sometimes sold as shai hong. Because it is sun-dried rather than fully fired, it behaves somewhat like raw pu-erh and does change over years. It is a niche product, and most red tea in Western shops is not made this way.
Everything else, including expensive single-origin red teas, is a short-term proposition. The better the tea, the more aroma it has to lose, and the more you lose by waiting.
This is the opposite of how most people treat expensive things. With red tea, the good stuff is the most perishable, not the least.
Which is your case
Which case you are in is mostly about volume. If you drink red tea daily and finish a 250 gram tin in two or three months, you are a short-term storer and the container is the only thing that matters.
If you buy a kilo at a time and take two years over it, you are still a short-term storer, but you are doing it badly. Split the purchase into three or four sealed tins on the day you get it.
If you are genuinely thinking in years, buy dark tea instead. That is the category designed for it, and storing red tea for a decade produces something dull and slightly sweet with no distinguishing character.
And if you have inherited an old tin, the answer is not to store it better. It is to smell it, brew it, and accept that it is what it is.
The failure mode for red tea deserves a note, because it is unusual. Red tea almost never goes off in a way that makes it unpleasant. It goes flat, which is easier to miss.
You can drink a tired red tea for months without noticing, especially with milk or in a blend. That is why the date on the tin matters more here than for any other category.
A flat red tea is still usable food. Brew it stronger, ice it, or use it as a base for chai. There is no safety issue, only a loss of character that no storage decision can reverse.
A quick decision rule
The rule for red tea: buy in 100 to 250 gram lots, split them into sealed tins immediately, and finish within a year.
Keep it in the dark, keep it dry, and above all keep it away from the kettle. Steam is what ruins red tea at home, not heat and not time.
If you want to age something, buy sun-dried Yunnan red deliberately and treat it like raw pu-erh. Ordinary red tea will only get flatter.
Date every tin. Red tea never announces its decline, and without a date you will drink a two year old tin without ever noticing that it used to be better.
One last practical point. If you buy red tea in bags rather than loose, keep the bag inside a sealed tin rather than clipping it shut. A clip is not a seal, and the tea will taste of the cupboard within a season.