user: Question: What is the number of interregnum for duration 3 months, 6 days?
Columns: ['Interregnum began', 'Interregnum ended', 'Duration', 'Count Palatine of Saxony', 'Count Palatine of the Rhine']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['9 December 1437 death of Sigismund', '18 March 1438 election of Albert II', '3 months, 9 days', 'Frederick II, Elector of Saxony', 'Louis IV, Elector Palatine']
SQL:

assistant: SELECT COUNT("Interregnum ended") FROM "Table" WHERE "Duration" = '3 months, 6 days';