If a keg of beer is un-broached, that is opened, then it has a shelf life of 2 months. See the Best Before date on the keg label. You should be able to use it at any time within that period. Once a keg is opened however it should be sold within a week approximately. If a keg is on service for longer than this it will become over-carbonated and become lively and possibly develop poor flavours. As a rule of thumb you should be selling one keg of any product within a week, if not then you should review if it is profitable to sell that product. John Smiths is the same as all keg beers in this respect.