Once a keg of any beer is connected the clock is ticking. You should hope to sell a keg within 4 to 5 days ideally. After this depending where the keg is stored temperature is crucial, then you may find problems such as off flavours, frothing and in some cases infection within the beer setting in. The best before date describes the length of time within which an unopened keg may be put on sale. It does not indicate how long the beer will last once on service.