When brewed coffee sits out it can go bad, but just how quickly depends on several factors. For a full pot of whole bean coffee, you can usually drink it up to 24 hours after brewing it and it will still be pretty good. But for a cup of joe with milk or a milk alternative, it gets more complicated.
The main factor is oxidation, which happens when the aromatic compounds in your coffee come into contact with oxygen. These compounds degrade over time, and they are more likely to do so when your coffee is hotter. So if you want to keep your coffee as fresh as possible, it’s best to consume it within 30 minutes after brewing it.
Another factor is bacteria, which can start to grow in dairy products left at temperatures between 40 and 140 degrees Fahrenheit. The general rule of thumb is to toss your coffee if it’s been sitting out for two hours or more, since this is when bacteria starts to build up and cause illness.
Cold brew is an exception to this, as the process of oxidation happens much more slowly in this type of coffee. In addition, the acid in cold brew can also help prevent the growth of bacteria. This makes it a safer option if you want to store your coffee at room temperature for longer. But you should always try to store your brew in an airtight container so that it will last as long as possible.