Where does Drupal save the trim length for the body field? When I navigate to Manage display for any node type that has a standard body field and click on the Teaser tab the default format will be "Summary or trimmed". The default trim length is 600 (characters), but you can change it to whatever you want and Save the new value.

However, I want to know where (i.e. in which table or variable) this value is saved.

(I need this information in order to find out what trim length to use when comments are emailed to users that subscribe to comment notifications. The project specification requests comment notifications to be trimmed to same length as the content type they're attached to.)
In Drupal 7, field settings are saved into {field_config_instance} and {field_config} database table.

Trim length for a body field is saved in the data column in the {field_config_instance} table.  The data column is stored in BLOB format, so you are not able to see the value of this. 

You can use the field_info_instance() function to extract information about a particular field in a particular entity bundle/type:

$body = field_info_instance('node', 'body', 'page');  
print_r($body);