There is NO specific frequency for updating an Entity Subtitle.
Revise your Entity Subtitle when there are significant changes to the brand. Examples include changing professions or rebranding, or repositioning. Update the Entity Subtitle if the change could confuse Google’s algorithms and its understanding of the entity. Updating the Entity Subtitle requires consistent changes across all your online profiles and mentions. This is an extensive task.
Regularly reviewing the consistency of information across all profiles and digital presences is a best practice for brands. Even if the Entity Subtitle doesn’t change this helps to ensure that Google’s algorithms understand the entity accurately.