Temperature
Red tea, the category most of the English speaking world calls black tea, wants cool and dry. Room temperature is fine, and refrigeration is unnecessary, because the tea is fully oxidized and has very little left to lose enzymatically.
What red tea loses is aroma, and heat accelerates that. A tin kept at a steady 20 degrees will hold its character noticeably longer than the same tin kept at 28, and the difference shows up within a year.
Stability matters more than the absolute number. A cupboard above the oven, which swings twenty degrees twice a day, will flatten a red tea faster than a slightly warm room that never changes.
The corollary is that a cool room is worth finding, but not worth obsessing over. Red tea is forgiving, and the mistakes that actually ruin it are steam and a loose lid, not a few degrees of warmth.
Humidity
Keep it dry, below about 50 percent relative humidity if you can measure it. Red tea is dried to a low moisture content at the factory and does not need any humidity to do its job.
It also does not usually mould. Red tea is the category least likely to grow anything, and the failure you should actually watch for is the opposite one: the leaf going dull and brittle, and the smell going flat.
The realistic risk is steam. A tin kept beside a kettle or above a hob will pull in moisture every time someone boils the water, and over a few months that is enough to make the tea taste stale and slightly sour.
Light and air
Keep it dark. Light fades the leaf and thins the liquor, and it does it faster than most people expect.
Keep it sealed. Red tea is rolled, not pressed, so it has a lot of exposed surface, and it gives up its aroma readily to any container that is not genuinely airtight. A tin with a loose lid is barely better than a paper bag.
And keep it away from smells. A red tea stored beside spices or coffee will take those notes on, and because red tea is usually brewed strong and sometimes with milk, small off-flavours are easy to miss until they are obvious.
The target environment
The target is a sealed, opaque tin in a cool, dry cupboard that does not sit near the kettle or the oven. That is the whole specification, and it is easier to meet for red tea than for any other category.
The one habit worth adding is portioning. If you buy 250 grams, split it into two tins and only open one. By the time you finish the first, the second has barely been exposed, and it will taste fresher than any single large tin would.
Date the tin when you open it, not when you buy it. Red tea does not announce its decline, and without a date you will not notice that the tin at the back of the cupboard has been open for eighteen months.
Adjust through the year
Seasons matter less for red tea than for green, but they are not irrelevant. A summer heatwave that pushes an upstairs room to 30 degrees for weeks will cost you aroma, and that aroma does not come back in autumn.
Recheck the storage spot at the start of summer and again at the start of winter. If the room gets hot, move the tea downstairs or to an internal cupboard. Floor level in an interior room is usually the most stable place in a house.
Humidity swings are the other thing to watch. A damp winter in an unheated room can push a cupboard above 65 percent, and while red tea is unlikely to mould, it will go flat and soft. A hygrometer will tell you before the tea does.
If you are storing red tea somewhere genuinely damp, a food-grade silica sachet dropped in the tin costs nothing and solves it. Check it every few months. Most types change colour when they are saturated, and they can be dried out and reused.
What you should not do is try to rescue red tea that has already gone soft by drying it in an oven or a pan. That cooks the leaf, and the result tastes of cardboard rather than tea.