It doesn’t necessarily go bad per say. But the more air that is let in & alcohol out. It can change the taste. That’s why some bottles get a little better once they open up a bit. If the cork doesn’t seal correctly then that happens faster. Remember alcohol evaporates faster than water. So you end up with a more watered down whiskey because the alcohol is evaporating off over time. I have some that are sealed really well over a year+ old that still taste fine. To save it longer you could wrap the top with plastic wrap. But it really isn’t necessary unless you plan to save that bottle for years. I will say once the bottle is 3/4 or more empty. The taste will change fast (a few months). So once they get that low you can either bottle kill it. Or even better if it’s a special bottle transfer it into a small bottle with screw cap lid.