Decanting oxygenates the wine, making it taste brighter and aromatic. The amount of decanting time varies depending on the wine. Generally, 2 to 3 hours is the most you would want to decant a wine for before serving. However, unlike whisky, wine should not be left in the decanter indefinitely; 12 hours is the max. You can keep opened wine for about 3 to 5 days but that wine needs to be stored in the re-corked bottle (whether the original cork or a wine stopper) in a cool dark place, such as your fridge. Generally sweeter wines will keep longer than dry wines.
Unfiltered wines should definitely be decanted as there is a good chance there will be sediment in the bottle. Most wines are filtered but some are not. It would be helpful if unfiltered wines stated so somewhere on the label but I have found that you can’t count on that. If there are vintner notes for the wine, those will note if the wine is unfiltered.
Unfiltered wines should be passed through a strainer when being poured into the decanter to catch the various bits of stem and grape skin.
Young wines don’t require it because they are already full of oxygen and aroma but older wines need to be decanted if you want to experience the aroma. However, as a rule of thumb, it is recommended that a wine that is 20 years old or more should not be allowed to decant before serving. In this special situation decanting would cause the wine to lose some of its bouquet and flavour. That being said, it is a good idea to filter these wines as they are being poured since they will most likely contain sediment that will have accumulated during the extended aging process.
Wine decanters themselves come in a variety of shapes, sizes and price points. Most, like wine glasses, will be widest in the base. This allows for the most efficient oxygenation to occur. Unless you want your decanter to double as a display piece, you don’t need to spend a lot of money on one. Your local kitchen or home décor store should have a good selection to choose from.