How long will an open bottle last before going bad?
I'm sure that I'll get flak for saying this, but.... keep the bottle well sealed standing upright on your fridge door shelf. In the fridge, it'll last a *long* time before "going bad", like sherry. I use quotation marks, because it won't go off the way table wine does. It will lose some of its complexity, and will be a less pleasant beverage, as it ages in the fridge, but I'd say that you're good for a few months, at least. Another suggestion is to experiment beyond veal marsala. I love zabaglione, and have used marsala (granted, not dry) in rice puddings and pound cakes. You could use it to deglaze a roasting pan for jus, or in a gravy. Sprinkle it with some fine sugar over berries. A pleasant preprandial cocktail is marsala, a spritz of soda water, and a twist of lemon. You should be finished the bottle in under two weeks, with all those suggestions! Cin cin!
Most wines go *bad* after opening because they become oxidized. Marsala is a wine that is *already* oxidized, and it's that oxidation that makes Marsala taste like Marsala. It will take a very long time indeed for it to go bad. You could probably keep it sealed and refrigerated for at least a month, probably longer.