This is the actually shelf life of the keg untapped. When tapped, the only change that will occur other than natural flavor deterioration will be carbonation level. This is why temperature and pressure is critical. It is very difficult if not impossible to gain perfect gas equilibrium between the carbonation in the product and the CO2 used to propel the beer during the shelf life. The key is to get as close as possible while not changing the carbonation level of the product which can change the flavor and cause dispensing issues.
Scott Zuhse, Instructor
Micro Matic Dispense Institute