As stated, keg-life depends on what kind of beer you’re working with. For draft beer in general, the life expectancy for pasteurized beer will last about 3 to 4 months, or 90 to 120 days. Unpasteurized draft beer is considerably shorter, only lasting about 6 to 8 weeks, or 45 to 60 days.
The life expectancy of your keg starts the day it’s filled at the brewery. Look around the keg for the date. It should be the “birth date,” meaning the day it was filled, or the expiration date. If the keg is passed the expiration date or you’re not sure whether the beer is pasteurized, throw it away.