It depends, to a certain extent, upon the ratio of air to wine. I opened a bottle of red on Friday evening and only drank about a quarter of it. I drank the rest last night (i.e. after 4 days) and it was OK. However, if I'd have drunk three quarters of the bottle on Friday, I very much doubt that the remaining quarter would have been drinkable much after Sunday evening.
Although unopened wine should be stored with the bottle on its side (because this helps to maintain an air-tight seal), a bottle which has been opened (with the cork replaced) should always be stored upright. This is so that the surface area of the wine, which is in contact with the air in the bottle, is kept to a minimum.
If you want to keep wine for longer, I suggest buying one of these:
http://www.amazon.co.uk/Vacuvin-Vacuum-Saver-G iftpack-Stoppers/dp/B0000AQVO2
(You don't have to buy from Amazon. Lots of shops sell them). I used to run a sports club bar where we had very slow sales of red wine. By using one of these vacuum pumps, an opened bottle could be kept for several weeks.
Chris