The main problems are heat, light (especially ultra violet), certain heavy metal compounds such as ferric iron or cupric salts, and of course oxygen. However, there is no need to be too obsessional. Under reasonable conditions a few minutes one way or another would do little harm. Even jams and similar foods prepared under reasonable conditions will contain useful quantities of vitamin C. Keep things simple and keep them clean and don't prepare juice for consumption before it makes sense to do so. That is usually all you need.

Enjoy.

Jon

 Oranges and orange juice are famous for their hight vitamin C (ascorbic acid) content. Vitamin C is a water-soluble vitamin that is easily destroyed by cooking and freezing. Now, going back to the question, the vitamin C content of orange juice is less in the juice that has been exposed to the air, but not because it has evaporated. Vitamin C is one of the most unstable of the vitamins, and it is readily oxidized when exposed to the air.  This oxidation converts ascorbic acid into a form that the body cannot use.  If orange juice is covered and stored in the refrigerator, there will not be much loss of vitamin C. However, if it is stored at room temperature or uncovered, more of the vitamin will be oxidized and the juice will, in a couple of days, contain very little vitamin C. Therefore you do not need to drink all the juice in one goal, if the juice is stored well, it can be consumed in around three days.