How long can wine be stored at room temperature once it has been opened?

Not long. Some old, fragile wines begin to deteriorate immediately upon opening. Although I haven't experienced it personally, I have been told (and I've read) that very old wines will actually noticeably fade within minutes of opening. In other words, the first glass may taste great, but that's it. But that is an extreme case. You're talking about everyday wines. It has been my experience that opened bottles of just about any red wine will deteriorate in a day or two if left out. Placing them in the fridge doesn't seem to help much. The deterioration in primarily due to oxidation, which colder temps don't do much to reduce. I have had some success, however, with those vacuum pumps closures. Wines seem to taste better the next day, and even the day after that, if I use the pump closures, which evacuate air from the bottle and reduce the oxidizing effect. White wines seem to do better than reds after opening, and jug wines, even reds, seem to hold up pretty well, also.