Kombucha mushroom will take the shape of the mouth of the jar and keep growing in thickness with every new batch. It is time to trim it when SCOBY is too thick and looks like a mothership, has many thin brown layers at the bottom and even holes. Remove weaker bottom layers, peeling them off by hand. You want to keep a few white thick gelatin top layers for next brewing session. You can also use a pair of clean kitchen scissors to cut down the amount of SCOBY in half and in any direction, to allow room for more kombucha to be brewed and proper airflow. You can trim it every few months or every few brews, up to you.