How long can beer sit in a keg?

[ShowPonyBrewing]

Ok. The beer I just kegged was done by popping the lid and filling thru the opening, then I force carbed it and purged the head space so I’m assuming that will be sufficient. I do like the idea of a closed system transfer to keep out any o2.

 

[chickypad]

Actually, that is how it works. At least for the short term. Introduction of co2 will initially stratify and then eventually will diffuse.

http://beerandwinejournal.com/can-co2-form-a-blanket/

He is talking about a stream of CO2 that you are infusing from the bottom of the vessel, and makes the point that it will rapidly diffuse unless you continually replenish it but might give you a little protection. This is not the same as purging a keg a few times from your gas inlet, if you are envisioning that gas floating to the bottom in a blanket it's just not happening. There will be less oxygen there it was before, but it's still a fair amount of oxygen and not nearly as good as purging the whole keg with a transfer of star san (hoping @doug293cz will chime in with some data for you!). It's all a matter of what degree you want to go to. Many people are content to just rack quietly and then purge a couple times. Others fully purge their kegs and do all closed transfers with CO2. Then there are the LoDo brewers who even avoid all hot side aeration carbonate naturally with spunding valves to avoid the tiny amounts of oxygen that could be present in bottled CO2.

 

[Brewsit]

Yea it’s all relative... this is one of those hobbies that have no limit to how particular you want to get...

 

[Dog House Brew]

you could natural carb your kegs..

If I were planning Long storage I would not force carbonate. I would naturally prime the keg with some sugar, seal lid w CO2, let set 2 wks room temp, then keep cold. Force carbonating will introduce too much O2 IMO. There are calculators for carving kegs. I would fill the kegs all the way to gas tube to limit your headspace.

 

[Cavpilot2000]

"How long can beer sit in a keg?"
A couple hundred years, I would imagine, until either the keg rusts through or the seals deteriorate enough that the beer is able to evaporate.

Your real question is more likely "How long can beer sit in a keg and stay good and fresh-tasting?"
As others have said, it is dependent on the degree to which you can reduce oxygen exposure.
If it is already fully fermented out, you can add the priming sugar, wait for yeast activity, then closed loop transfer to a purged keg (via purged lines).
Alternately (and preferably), you time it so you have about 4 gravity points left to full attenuation and transfer the beer (via purged lines to a purged keg) and let it naturally condition with its own residual sugar (called spunding - do some searches, there's lots of info on this). By the way, you don't use an airlock here - you can use a spunding valve or just leave it sealed. This will also create the pressure you want to help keep air out. This is preferable because you are naturally carbonating the beer without opening it up or adding additional sugar. You'll also get better foam and head retention on a spunded beer.
Give it a try.

Then storing it as cold as your logistics allow will help (once you have given it a week or so to finish fermenting and carbonate). In a fridge (lagering) is ideal, but a cool basement or some such will do. The cooler the better.