Coffee can sit out for at least 24 hours before it starts to grow mold in it.
I've left coffee in my coffee pot all night and then reheated it the next day and it perfectly fine to drink although it got stronger in taste.
I just added some additional water to dilute the stronger coffee and heated the coffee in the microwave and it was okay to drink.
It wasn't the freshest coffee but it was perfectly safe and okay to drink it.
If you leave coffee out longer than 24 hours then it starts to go bad and may have mold in it.
Just check to make sure the coffee hasn't grown any mold in it and if not then it's safe to drink.