title_body: How long does it take before the taste 'flies away'
upvoted_answer: I would say that it has to do more with carbonation than anything else. Like soda, beer goes flat after a while and tastes bad (or not as good) when the carbonation is gone. Some beers have more carbonation than others, so I don't think you can accurately measure this, it really depends on the brew.
  
  Oxidation (exposure to air) also may negatively impact the flavor. Of course, I wouldn't recommend exposing a beer to air for weeks and then drinking it due to various health concerns and I can't say I've ever tried it, nor do I want to.