expiration time for a cup of coffee

For best flavor drink the coffee within 20 minutes of brewing. As far as adverse health effects from drinking coffee that's gone cold sitting on your desk, I'm not aware of any unless you've put milk in it. You can drink black coffee at 3pm that was poured at 8am though it might be unspeakably foul tasting. If you're really worried about it, just follow that old food service industry rule "When in doubt, throw it out." Dump that cold, nasty coffee and get (or make) a fresh one.

I refuse to waste "perfectly good" coffee even if it's a half-mug left over from much earlier in the day. My solution? Fill the empty space with chocolate milk and then nuke it. Presto! A reasonable facsimile of mocha (or whatever the correct term for a coffee/chocolate mix is).

Waste not, want not. Works for me...

Curmudgeon

Because of my work situation, and liking good coffee, I've gotten used to drinking coffee after it has gotten to room temp.

Long story short, I would rather drink room temperature black coffee than coffee that has been on a warmer for more than about 20 minutes!

As far as how long it is "good"? Well, if it is black, nothing is going to spoil for a looong time. Hours, for sure, maybe days.

As to what is better, a freshly brewed cup, or one that has gotten down to room temp -- no question, fresh is better!

You might want to see about getting a Nissan Thermos mug--one source for most of the line is . Keeps coffee hot or soda cold for a remarkably long time.

I use both a Nissan Thermos mug and a 1-liter insulated bottle. The mug will keep things hot for about 4-5 hours while the bottle will keep the coffee hot all day. The flavor changes throughout the day, but it's still a lot better than whatever is in the office carafe burning on the warmer.