Often time’s people are befuddled by the process of decanting. Knowing your wine is an important part of it. Has the wine been filtered or not? How old is it? How has it been stored? Is it made from thin skinned or thick skinned Varieties? All of this plays a role in the decision to decant or not.
Wine is decanted for 2 basic reasons; the first is to aerate and the second is to remove any sediment from the wine. The technique is basically the same no matter what the rationale is and may be appropriate for white wine and in some cases even sparkling wines.
Older wines will have developed a bit of sediment in the bottle as the suspended particulate matter re-solidifies in the bottle. Sediment is not harmful to consume but it should be carefully separated out from the bottle. This is done by carefully removing the bottle from the rack and placing it in a cradle on its side. The bottle remains in this position as it is carefully opened. Next place a lit candle in between the wine cradle and the decanter. Slowly remove the bottle being careful to keep the bottle on its side without spilling and delicately pour the wine into the decanter. Look down over the shoulder of the bottle using the candle as an under-light. Stop pouring when you start to see the smoke entering the neck of the bottle. If done properly there should only be an ounce or 2 of wine left in the bottle. Older wines are more delicate and depending on the variety, age and storage conditions, may need to be drank rather quickly before it starts to fade.
Aeration is generally done to younger wines that need some extra oxygen exposure to open up the aromatics in wine. Many young wines will not have any noticeable sediment so this is a good place to practice your technique. Under these conditions the wines will benefit from 30 min to 24 hours of time depending on the wine. A young new world wine like syrah will require less time than a young Barolo will.