Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_34 ( duration VARCHAR, start VARCHAR, stage VARCHAR, difficulty VARCHAR )

### Question ###
How long does the one that stars with son bou of middle difficulty, and on a stage number larger than 13 last?

### Accurate SQL ###

SELECT duration FROM table_name_34 WHERE stage > 13 AND difficulty = "middle" AND start = "son bou"