How long does a bottle of wine last after opening?
First things first, pop the cork back into the wine and put it in the fridge. Yes, and that goes for reds too, most of the time.
White wines or rosés can last up to three, even four days. I've found that it can depend on the type of wine though, I've had some last a good six days, particularly those that come with a screwcap rather than cork.
Sparkling wines can keep while maintaining most of their fizz for three to four days, but it's worth sealing them with a special sparkling wine stopper. And no, a spoon won't do the job for you. I've no idea where that myth came from.
Reds can last a similar amount of time. I've left them out of the fridge for three to five days, keeping the cork in them and storing them in a darker, cooler part of the kitchen. Over two days, many will actually get better! I've found the fridge does a great job with reds too, particularly the lighter styles like Gamay or Pinot Noir. Bigger, richer wines tend to last a day or two longer than lighter wines. And older wines can 'fall apart' a bit after two days.
And keep your wines out of the fridge unless you're cooling them just before opening. The cold can harden the cork and lead to oxidation. And, vegetables like cut onions can impart aromas you just don't want in a wine!
You can invest in corovin or similar systems which inject a layer of inert gas into the bottle. To be worth the investment, though, they'd need to be fairly special wines.
Lastly, if you know you're not going to drink the wine, then freeze it in ice-cube moulds - great for adding to sauces when the need arises.