Table: vicarship_data
Columns: interregnum_began
interregnum_ended
duration
count_palatine_of_saxony
count_palatine_of_the_rhine

Question: What is the number of interregnum for duration 3 months, 6 days?
SQL: SELECT COUNT("interregnum_ended") FROM vicarship_data WHERE "duration" = '3 months, 6 days'