I think bottled beer can last way longer than 6 months. Some of the beers I make take that long to condition and mature in the bottle. I really depends on the beer more than anything else.
I've found that low alcohol hop forward beers, think IPA's, are best when they are young and the hop flavors (the aroma hops more than anything else) are at a peak. This occurs when the beer is young. Older beers, those more than 6 months old, will lose a lot of the aroma hop additions. This isn't to say they turn stale or are undrinkable. The aromas simply degrade and are not as noticeable as they once were.
Time has an immense impact on the taste of beer. In the case of home brewed beer this can often be a saving grace. Beer brewed with plenty of mistakes can, with age, become simply wonderful brews. I've scene more than a few of my beers go from ick-to-slick simply through the passage of time. In other words, beer I wouldn't give to my dog has transformed into some of my most coveted beers with time.
So how long does beer last in a bottle? If you are talking about home brewed beer, I'd say a very long time. I would say somewhere between years and decades. Depending on the style. High alcohol beers can easily go for decades and be developing new and complex, and desirable, flavors the whole time. Even low alcohol beers can benefit with age. I brewed an awful beer many years ago that was 5% abv. 3 years later I'm still pulling out bottles to taste. 3 years ago you wouldn't be able to stand the smell, let alone the taste. Now, it isn't bad. It isn't great, but it was absolutely infected on top of being brewed in the worse conditions imaginable. The fact that even it is now palatable (its better than Bud-Light), says something about how age and home brew compliment each other.
Don't be fooled or buy into the "freshness dating" marketing of the big brewers. Their beer may need that, after all, they use over 200+ ingredients for their beers. Home brewed beer is a completely different animal. It is natural, it is good, it doesn't have more ingredients than you have fingers. And as such, it keeps for a very long time. As long as you have a solid air-tight seal on the container, don't worry about it. Time will not harm it. It might change it, but it won't harm it.