A long time! When stored in the refrigerator, it should maintain its quality for up to 9 months. When you store homemade vinaigrette in the fridge, the oil will solidify (store bought dressings have water and gums added to avoid this natural reaction) and you will need to warm it a bit to mix it and serve. TIP: If you'd like to avoid that, it's perfectly fine to store your oil-based dressings at room temperature for a couple weeks, as long as there are no fresh ingredients like herbs or garlic. When adding fresh ingredients, the vinaigrette should be refrigerated.