First and foremost, coffee’s flavor and freshness are paramount. Freshly brewed coffee tastes best when consumed right after it's made. However, if you have a leftover cup, here are some important factors to consider:

1. Type of Coffee: The type of coffee you’re dealing with plays a significant role in how long it can sit out. Black coffee, which is devoid of milk or cream, can generally last longer than a latte or cappuccino, which contains dairy.

2. Room Temperature: The temperature of your environment can also affect how long coffee can sit out. In a warm room, bacteria can grow faster, reducing the quality and safety of your coffee.

General Guidelines

1. Black Coffee

• Room Temperature: If left at room temperature (around 70°F or 21°C), black coffee is generally safe for about 30 minutes to 2 hours. After two hours, it may not pose a health risk but will start to taste stale and bitter.
• Refrigerated: If you put your black coffee in the fridge, it can last up to 3-4 days. Just be aware that the flavor will continue to degrade over time.

2. Coffee with Milk or Cream

• Room Temperature: If your coffee has milk or cream in it, it’s best to consume it within 2 hours. After this point, bacteria can start to grow, making it unsafe to drink.
• Refrigerated: When refrigerated, coffee with dairy can last 1-2 days. Just like with black coffee, the flavor will deteriorate, so consider reheating it gently before drinking.

Signs Your Coffee Has Gone Bad

It’s crucial to use your senses when determining whether your coffee is still good to drink. Here are a few signs that your coffee might have gone bad:

• Smell: If your coffee has an off or sour smell, it’s best to avoid drinking it.
• Taste: If it tastes stale or rancid, that’s a clear indication that it’s time to discard it.
• Appearance: Any visible mold or curdling (especially in coffee with cream) is a definite red flag.

Storing Coffee for Freshness

If you want to extend the life of your coffee, here are some best practices for storing it:

1. Use an Insulated Carafe: Instead of leaving your coffee on a hot plate, which can burn it, transfer it to an insulated carafe. This will keep your coffee warm without affecting its flavor.

2. Store in the Fridge: For leftover coffee, especially black coffee, consider storing it in an airtight container in the fridge. Just remember, reheated coffee won’t taste the same as freshly brewed.

3. Freezing: If you have a significant amount of brewed coffee that you can’t consume in a few days, consider freezing it in ice cube trays. You can use these coffee cubes later for iced coffee or in recipes.