How Long Is A Keg Good For Once It's Been Tapped? - Tasting Table
Briefly

Draft beer is generally fresher than bottled or canned options, leading to a superior taste, but freshness depends on factors like pasteurization and storage.
Kegged beer freshness varies: pasteurized beer lasts 90-120 days, while unpasteurized beer should be consumed within 45-60 days, depending on storage conditions.
Using CO2-based dispense systems can preserve freshness like unopened kegs, whereas manual party pumps introduce oxygen, leading to stale taste within hours.
Cask ale, kegged with live yeast cultures, can turn stale quickly in just three days, contrasting with traditional kegged beer.
Read at Tasting Table
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