There is no singular answer to this question. In general the aiming masks should be replaced when:
•Between each relay during a competition. The front aiming mask may be reused if it was not shot up. The back aiming mask should be replaced between each relay regardless.
•At the start of each practice session, if the previous front aiming mask was shot up.
•In BB Gun, the front aiming mask may need to be replaced after standing, as most athletes will shoot at least a few shots into the white in this position.