How long is a growler good for?

Ive always been told that at the most they are good for 3-4 weeks if you dont open them but that once you open them, you have a day or 2 to drink it.
Ive considered starting to buy local craft beer in growlers because it is a bit cheaper than buying bottles but I dont feel that I really drink enough beer for it to be worthwhile because Id end up dumping half of it out after it goes flat (and Id hate to waste good beer like that).

 

I had a swing top of 11% abv. beer last 3 months in the fridge.

 

Just curious where would you get a growler of BCBS? Never even occurred to me that that was a thing.

 

Standard bottles have a CO2 purge as part of the bottling process or are filled in a chamber where no oxygen is present. Oxygen/Light are enemies of freshness, if you don't get the oxygen out of the growler, the beer will oxidize and lose flavor.

 

I filled a couple of jugs from my keg I had last year. I figured they would last.

 

@reefer_bob already addressed the oxygen side of the bottling process. The other difference between a typical growler fill and bottling is counter-pressure filling. A bottling line will fill via counter-pressure filling which keeps the CO2 in solution during the filling process.

There are super-duper growler filling stations made my Alfred Gruber which does fill a growler just like a bottling line. The growler station performs a double evacuation using CO2 and then fills via counter-pressure filling. Below is a video of the growler station at Iron Hill Brewpub (Maple Shade, NJ):