This really depends upon the media, environment, cycle rate and type of knife gate valve selected. If the media is clean and the valve is not cycled that often the lifetime could well be decades. If it is a corrosive environment or something that will wear the seat out the life will be much shorter. Other features such as deflector cones can also be added to enhance the life of the valve whereby they defelect media away from the seat of the valve when it is opened.