Generally in a situation like that a restaurant or bar will order very little of their least "popular" beers.
As with any food or drink, freshness is important, and beer is no different. Of course it depends on the beer and how it is bottled/canned and stored. a good rule of thumb would be that a cool dry place or refrigeration will hold a beer for anywhere between six months and a year.
restaurants are going to have the advantage of reviewing past sales data of course, so they are able then to order more of the beer that sells and less of the beer that doesnt.
it is best to keep it refrigerated, or in a storage area that is not subject to temperature changes. If for example your basement or store room holds a pretty constant average temperature without much fluctuation - that is the best storage conditions
EDIT: i will add, that beers with a higher % of alcohol content will shelf for longer periods of time than lite beers with a lower % of alcohol content.
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