A decanter is not a ceremony and it is not a fix. It does two specific jobs, and knowing which one you need decides how long the wine should sit in it, and whether it should go in at all.
Two different jobs
The first job is separating clear wine from sediment, which matters for old reds, unfiltered wines and vintage port. The second is exposing a young wine to air so that tight tannins and closed aromas loosen. They are not the same task, and the second one can spoil a wine that only needed the first.
Decanter puts rough timings on the aeration job: mature and lighter-bodied wines can open up within ten or fifteen minutes but may start losing aromas after about an hour, while young, tannic, full-bodied wines can take an hour or more, occasionally four. More than two hours is risky for most wines.
Sediment: stand it up, pour in one movement
Stand the bottle upright for a day or two before you plan to open it so the deposit falls to the base. Then pour slowly and continuously with a light behind the neck, and stop as soon as sediment reaches the shoulder. The last centimeter is the price of a clear glass.
Old bottles are also the ones most likely to have a fragile cork. If it crumbles, the opening guide covers getting the wine out without pushing cork through it, and a coffee filter or fine sieve rescues the pour.
Air: how long, by style
Young, tannic, full-bodied red: 1 hour or more Young everyday red: 15 to 30 minutes, or none at all Mature red, robust: 15 to 30 minutes Mature red, delicate: pour and taste, often none White, aromatic or reductive: 15 to 20 minutes Vintage port and unfiltered reds: for sediment, then taste More than 2 hours: rarely helps, often hurts
The bottle is its own test. Pour a glass on opening, taste, and taste again twenty minutes later. If the second glass is better, the wine wants air and you can decant the rest with confidence. If it is worse, you have learned something more useful than any chart could tell you.
What does not need decanting
Most whites, most roses and most everyday reds gain little. An old and delicate wine can lose the light aromas that make it worth drinking, sometimes within minutes, which is why some experienced tasters refuse to decant mature bottles at all. If a wine has spent thirty years getting to this evening, treat the first glass as the experiment.
One case is worth knowing: a wine that smells of struck match or drains on opening is often reductive rather than faulty, and air is exactly the remedy. The fault guide covers telling that apart from cork taint, which air will not fix.
If you do not own a decanter
Any clean glass jug works, and so does pouring the wine into a second bottle, which is how double decanting gets its name: into the jug for air and sediment, back into the rinsed bottle for the table. A large glass, swirled, is a small decanter for one person.
Serve at the right temperature after decanting rather than before. A wine sitting in a warm room in a wide vessel warms faster than one in the bottle, and the serving temperature chart covers where it should land.
Record what the air did
One line is enough: better after an hour, or best on opening. That note is buying advice for the next bottle of the same wine and it is the kind of detail the tasting note format is built to hold.