Assuming decent sanitation procedures, and reasonable techniques, it can stay 'good' for years. It will change over time, but =can still be a decent beer.
Hop flavor and aroma will fade with time, starting as early as a couple of months, but in a well made beer, it will just change the beer rather than deteriorate it.
If you introduced oxygen due to poor procedures, it will stale over time (wet cardboard). Younger is better in this case.
If you bottled with a lot of yeast, you can get flavors from autolysis as the decays.
Higher strength beers tend to last longer.