Use it as long as you want to keep your beer warm. If its the brew belt, they recommend you not use it for more than 8 days with out letting it cool. If fermentation is over the temp isn't as important, but I would try to keep the beer above 64 just to make sure if the yeast aren't finished, they will still do their job and not go to sleep on you.
Bass Clef Brewery
Drank: Amber Ale
Drank: Nessy's 80 Shilling Ale
Bottled: Yellow Paddle Pale ale
Bottled: Ed Wort's Apfelwein
Primary: William Henry Harrison Cider
Planning: Cheese's Vanilla Caramel Cream Ale