I’m guilty of drinking my leftover morning coffee in the afternoon. I just grab it off my desk and heat it up and there’s my afternoon pick-me-up. Turns out, that may not be the best idea since I use flavoring in mine.
Yahoo writer Leslie Stephens researched the issue and says if, for example, your coffee had milk in it, you should probably drink it when it's fresh to be safe. Additionally, the coffee flavor can be affected as time goes by.
Some coffee can start to taste bad after the coffee cools down but otherwise you probably have about a four hour window before the oils in the coffee start to go bad, further altering the taste.
And if you thought you could just zap any bacteria that may have formed by microwaving your coffee, you should know it is possible, but you'll have to be really careful to heat the entire mug of the stuff all the way through.
Long story short: black coffee should be safe to drink later in the day but it might not taste so great. If you insist on drinking it, be sure you microwave it all the way through before drinking it. (Yahoo)