It'll be fine for a long, long time. Just think of a keg as a giant bottle. You can age a "big" beer in it for a couple of years, but you'd probably want to drink most other beers sooner. Not because they'd go bad, but because they'd be past their peak. Most regular drinking beers are best about 2-6 months after brewing.
Broken Leg Brewery
Giving beer a leg to stand on since 2006