How long can you use frying oil?

When things like potatoes,vadas donuts, and other similar items are fried it is safe to reuse the oil. When frying meat, chicken and fish, it is best not to reuse it. Also, the strong tasting foods like onions, spices, or garlic will create a smell in the oil, and it will not be ideal for reuse when frying sweets or any other items.