It depends on exactly what herbs or feeds are used & if they are in oil or vinegar. Some store-bought products as only for display. But the ones made for consumption that are professionally bottled & sealed will last about a year or so, until the are opened. Once opened, they need too be refrigerated & will last 2 to 3 weeks. If you make them yourself, they need to be refrigerated once the oil & herbs (or oil & garlic, etc) meet & will stay safe to use for only a few weeks. Vinegar with herbs (or garlic, etc) will last a longer than the fancy oils, but Refrigeration is still VERY important!
Shelf life is super important with these items because you can get food poisoning (botulism) if you're not careful.
Read Below (info from the internet):
Commercial garlic-in-oil mixtures are acidified to prevent bacterial growth. Most of these store-bought products can be stored safely at room temperature look for storage instructions on the label.
Unfortunately, do-it-yourself acidification of homemade herb or vegetables-in-oil mixtures is risky because not enough research has been conducted to know how much acid is needed to prevent bacterial growth. These low-acid foods can be a source of Clostridium botulinum bacteria.
Clostridium botulinum produces a toxin that causes botulism, a cause of fatal food poisoning. Because these bacteria are found naturally in soil, water and air, they could be found in any low-acid food and begin to grow and produce toxin when the conditions are right. That's why it is crucial to keep food safety in mind when storing your own low-acid vegetables, such as garlic, mushrooms and chili peppers or herbs in oil.
Take the following steps to ensure food safety when making and using your own homemade herb, vegetables and garlic-in-oil mixtures:
-Refrigerate fresh vegetables or garlic-in-oil mixtures, and don't keep them any longer than three weeks. After three weeks of refrigeration, botulinam toxin could have formed. Remove the vegetables after flavoring the oil, and the bacteria will not have a food source for growth.
-Use dried vegetables, garlic or herbs to flavor oil. These do not contain enough water to foster bacterial growth. Dried vegetables, garlic or herbs-in-oil mixtures can be stored safely at room temperature. Refrigeration may delay rancidity.
-Tomatoes-in-oil mixtures are safe, as tomatoes are high in acid and will not foster botulism-causing bacterial growth. Refrigeration may delay rancidity.
-Don't use vegetables-in-oil mixtures that show any signs of spoilage, such as bubbling or cloudiness.
For more information about making holiday gifts, visit OSU Extension's Lane County office Master Food Preserver Web site at http://extension.oregonstate.edu/lane/food-preservation/index.php or call the OSU Food Preservation/Safety hot line, open through Oct. 1. OSU faculty and trained volunteers staff the hot line at (800)354-7319 from 9 a.m. to 4 p.m. Mondays through Thursdays (except holidays).
In addition, the OSU Extension Service offers many food-preservation publications online or in print. Visit the online catalog at: http://extension.oregonstate.edu/catalog or call (800)561-6719 to request a printed catalog.
More Information (from Canadian food inspection agency):
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