A keg is just a bigger bottle. So, just like bottles, different beer styles and different ABV%, as well as temperature, all have an effect on shelf life. I've had imperial stouts on tap for over a year. Still yummy.
if its in a fridge they can last quite a while.. i'm sure longer than you would ever need to drink it.. i've noticed that most styles get a bit better after a couple extra weeks, then plateau to how its going to be for quite some time.. i've probably had some kegged for at least 3 months and no issues (though i only have experience with cold kegs)
A beer will age fastest at room temperature, and slowest at near freezing. Most people who want to age a beer (or wine) would prefer a steady moderately cool temperature like in a cellar.
In the fridge, a keg will age very slowly as a rule so if I have a keg that is a bit young and it needs a bit of age, I'll keep it at room temperature for a week or so (or longer, depending on what it is) before putting it in the kegerator. Hoppy beers will start to lose aroma fairly fast, but some beers will not change much at all over the course of their life in the kegerator.
Thanks for the responses guys. I figured temperature would help decide the age of the kegged beer.
Also, would there be any differences if you were to decide to bottle from the keg as opposed to bottling from the fermenter? Will the life expectancy of the bottles be any less in the beer bottled from the keg?