This depends on the style of beer, type of pump installed, whether that beer is pasteurized or not, and the temperature at which it's kept. Typically, an average keg of pasteurized beer will last at least 6-8 weeks before the taste is affected, and as long as 3-6 months at lower temperatures. Unpasteurized beer will generally not last longer than 2 months before losing its fresh taste, with temperature having less of an effect.