A good while if they're not tapped I imagine. Just a matter of knowing when it's worth putting a new barrel on or simply chucking a towel over the pump handle for that particular day.I am no publican, but how well do draught beers last in a barrel when they might not be used for 2 weeks? @peteblue
EDIT - doh, wot @Exeter_Gently said. Will know more about it than me.....which is true of a lot of people about a lot of things.