Q. One problem I encounter when cooking with alcohol (wine, sherry, etc.) is storing the leftovers after I'm done. If one is drinking wine, the usual advice is to drink it all at one sitting or to employ a vacuum-device or some other wine-saving device to squeeze a few extra days of life from the wine. However, what are the "storage rules" with respect to cooking sherry and cooking wine? Is it a bad thing to keep a bottle of dry white wine just for cooking that you use over 2 to 3 months?
Oh, you brute. First of all, you are not benefited by using cooking wine or sherry. They are not nearly as good as regular wines. Second, the salt in cooking wines and their general lack of quality do not improve if you keep them sitting around opened in the refrigerator for 2 to 3 months. Third, the food world expects you to cook, to cook well, and to cook with wine often enough that leftover alcohol is not a serious problem.
Actually, the food world is a bit unrealistic. No one expects you to use whole bottles of wine in your general cooking (yes, certain braising and poaching recipes do encourage you to pour liberally, but in most recipes, it's a few tablespoons or a cup or so). It wouldn't surprise us if many of the superheroes of the cooking world have a plugged bottle of wine in their refrigerators.
The problem with an opened bottle of wine is that exposure to oxygen causes it to sour, and, over time, turn to vinegar. Those with the most discerning taste buds will tell you this starts to happen very quickly. Those with average-Joe taste buds may not notice much difference for a while, especially if the wine is used for cooking and not drinking. It will help preserve the quality of your wine if you transfer it to ever smaller containers as you use it up, constantly decreasing the amount of air to which it is exposed.
If you won't use up a bottle in a week or two, you can freeze the leftover wine in ice-cube trays, and then transfer the cubes to a zipper-type freezer bag (from which you squeeze out as much air as possible).
We have been known to cook with a wine that was opened at least a month earlier and didn't have any complaints about the finished dish.