Yep - Coffee only keeps for 15 to 20 minutes before beginning to lose its flavor.

Why you should never percolate coffee:
Percolators violate most of the natural laws about brewing coffee.

Don't over extract the oils and flavor. Percolators work by taking coffee and reheating it and throwing it over the grounds over and over and over again.

Never reheat/boil coffee. This destroys the flavor. For best flavor, boil the water, pass it over the grounds and retain the heat. Don't reheat it.

Violating these rules may not sound like much, but these are about the only rules there are. The effect of a percolator is to keep passing boiling water/coffee over the grounds until there is no flavor left.

http://www.baristaguru.com/coffeebrewingtips.html