How long will wine keep in an open bottle?
How long will wine keep once the bottle has been opened? This may be the one wine question I hear most often; and the short answer, I'm afraid, is, "not very long." Wine, like fresh fruit, is perishable, and air is its enemy. Once you've taken out the cork and exposed the liquid to oxygen, it starts to deteriorate fast.
Wine shops sell a variety of preservation systems that suck the air out of opened bottles or squire inert gases in, but in my opinion, it's just about as effective simply to jam the cork back into the half-finished bottle. It will hold at room temperature for a day or two before its flavor starts to deteriorate seriously. Pop it in the fridge, and it might last for a week or more. Fortified wines like Port or Sherry may last a little longer, but much more than a week is pushing it.
Your best bet is simply to finish your wine within a couple of days ... use the leftovers for cooking ... or invite friends over to share.
One reassurance: Even if your wine gets too old to enjoy, it can't hurt you. It may lose its flavor and become flat, dull and unenjoyable, but it won't turn toxic.