Drupal 7: What's the schema of the 'data' column in the 'users' table?
There isn't a 'schema' as such, the values can be absolutely anything (that's why a serialised array is used rather than having statically defined fields).

From the user_schema() docs:


  A serialized array of name value pairs that are related to the user. Any form values posted during user edit are stored and are loaded into the $user object during user_load(). Use of this field is discouraged and it will likely disappear in a future version of Drupal.