If you are going to do it soon, then you can leave it out for a few days. But if not, i would just put it in the fridge. You can reheat it. I think it is often suggested over a double boiler, but you could probably do it slowly in the microwave or on the stove. Just be sure not to scorch it.
YUMM!! Ganache cakeballs!!
To answer your question... ganache will keep on the counter for a couple of days. You can also refrigerate it and heat it back up in the microwave on low power, watching closely and stirring frequently.
The "Art of Chocolate" book says if packed in an air-tight container truffles (ganache filling)will keep indefinitely in the freezer (leave in container to defrost to avoid condensation), 1 month if refrigerated. I think leaving it in the ganache form is best for the freezer.