user: Question: How long does the one that stars with Son Bou of Middle difficulty, and on a stage number larger than 13 last?
Columns: ['Stage', 'Start', 'Arrival', 'Duration', 'Difficulty']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Maó', 'Es Grau', '3 h 30 min', 'Middle']
SQL:

assistant: SELECT "Duration" FROM "Table" WHERE "Stage" > 13.0 AND "Difficulty" = 'Middle' AND "Start" = 'Son Bou';