The longest I've gone so far with a tapped keg is 6 months. None of the others have lasted that long. The quality of that particular brew, over that time period, stayed pretty constant with no noticeable signs of decreasing flavor, aroma, etc.
Like the replier above me stated, since it is constantly under a CO2 blanket, there is nothing really to make it spoil or oxidize. So unless you have a leaky keg or an infection in your tap line that makes its way into the keg, you are good for however long it takes to drink the beer.
Cheers!