If the beer was clean going in, and your sanitation was good, and you flushed the keg well with CO2, you should be able to age this for a loooooong time. Things like alcohol content and hops tend to perserve the beer a bit longer. What kind of beer is it? For something light and benign, you might not get as much time as something heavier and with more complex flavors. I wouldn't think 8 months is too long for any style, with longer times being possible for higher alcohol contents and higher hop content. I've heard of Lambics being aged for years, but if one of those spoiled, how would you know?
Barleywine is also typically aged for a year or more.
Try a small sample every 5 or 6 weeks and take notes to see how the flavor changes.