ive used the same cartridge many times over when re filling. i only normally move onto a different cart if i feel that its no longer peforming or if i notice that the wool has become very worn looking.
just pull the wool out, rinse under a tap, dry between a dish cloth or paper towel, replace in the cart, add your
juice and away ya go!
bob's ya uncle, fanny's ya aunt.
hope that helps!