The O-rings in the lid tend to last a year or so as they're pretty robust, but for ever other rubber seal and tap washer I change them after every three brews. The tap washer is especially important as it takes a great deal of pressure so theres a chance the washer will become cracked or warped on a fourth brew,which has happened to me, possibly due to too much pressure in keg . Better to be on the safe side, washers and seals are cheap.