depends on a lot of factors and your particular process... like how much total oxygen you allow in to the beer from start to finish, how much yeast and trub you let into the bottle, temperature its stored at, light intensity its stored under, the particular kind of beer it is...
generally the higher the alcohol content, the longer it will keep/the longer it takes to mature. hop oils degrade quickly, especially when struck by light, so an IPA looses freshness quickly.
if the beer has lots of yeast in it like a hefeweizen, autolysis also quickly reduces freshness, so those beers wouldnt last as long as a high alcohol stout or something.
filtering your beer and using oxygen-absorbing caps will extend the shelf life considerably.
