From my experience, I've never had one go bad and I just found one from three years ago that was still fantastic. On average, I tend to store excess beers in my basement (55-60F) for 1-6 months at a time. There are several different factors to consider: alcohol percentage, alpha acids present, strength of the brew, whether it's conditioned or force-carb'ed, sanitation practices, and storing conditions.
1) As a rule of thumb, the more ethanol in the beer, the better it ages. Ethanol present helps to ward off any would-be spoilers from growing, in addition to aiding in the development of character or the mellowing of the beer.
2) I've heard many times that highly hopped beers do not age well. I've never heard a good reason, but I can only guess that it's the gradual oxidation of the alpha acids that is being referred to, which gets beer all sorts of skunky. This can transformation can be prolongued by the use of oxygen barriers caps, which are lined with an oxygen absorbing material that helps to pull dissolved O2 out of the beer and away from the headspace. However, contrary to the name, oxidation does not require oxygen and will occur eventually all the same do to other compounds present in the beer.
3) Lighter beers don't seem to fair as well in the aging process, in my experience. A heavier, maltier brew will age better. If you're brewing lighter beers, chances are you'll be drinking them more readily anyway.
4) Forced carbonation will help drive O2 out of solution from the beer, potentially saving it from skunkiness, but the presence of living yeast helps with aging and will undoubtedly evolve the beer over time. Too much yeast is a bad thing, so be sure to bottle condition properly! Leave all the yeast cake and trub behind.
5) Sanitation is the most important thing in the history of the world, ever. At least for proper brewing. Well-sanitized bottles and caps are a necessity for aging, since even the smallest of small mold spores in a bottle will eventually destroy that bottle of beer. Between mold, wild yeast, bacteria, and even yeast-infecting viruses, sanitizing is crucial.
6) Ideally, you'd have a temperature-and-humidity controlled aging cellar with non-UV lighting at your house. Assuming you don't, which is probably a safe assumption, you'll want to store bottled beer in brown glass bottles away from natural light sources in a steady, 50-65F semi-dry area of your house. High temps can coerce even mostly dormant yeast to do funny things through enzyme activation, changing your beer flavor in strange ways. Too much humidity can cause caps to lose structural integrity, but too dry can eventually cause the seal to be compromised. Essentially, if you have a humidifier on your heater, you should be fine. Older houses may require a de-humidifier for basement/cellar storage. If it's too dank or too dry for you, than it's too dank/dry for the beer.
Hope this helps, and hope non of that info is misguided. Cheers, enjoy the brews!