How long will beer stay fresh in a keg?

How long will beer stay fresh in a keg?
Using standard tapper provided with keg at time of purchase... 3 to 5 days or less.Using a kegerator system with CO2 ... 2 months or longer depending on how cold your frige is.Air and heat is beers natural enemy. When using a hand pumped tapper the air forces the beer out nicely but it starts the cycle of the beer going flat. Rule of thumb "dont tap it until you can drink it all with in 24 hours"When using CO2 the beer will not go flat for months providing you have a properly functioning system. Real life fact - Over the last two years of owning a kegerator I can report that a half barrel of Millerlite will last two or three months (when pressurized by CO2 and kept at a constant 34f.) As you approach the 3 month point it will lose some bubbles and start to taste different. The clock starts ticking as soon as it was brewed and its best to try and get it as fresh as possible.