How often do the Black Berkey purification elements need to be replaced?

Each element has an expected life of 3,000 Gallons (11356 Litres) or 6,000 Gallons (22712 Litres) for a set of two. Usually replacement is suggested after 7-11 years. Filters needs to be cleaned every 12 months with warm water and a soft brush and primed.  (takes not more then 10-15 minutes)