What is the number of interregnum for duration 3 months, 6 days?
CREATE TABLE "list_of_imperial_vicars_1437_1792" (
  "interregnum_began" text,
  "interregnum_ended" text,
  "duration" text,
  "count_palatine_of_saxony" text,
  "count_palatine_of_the_rhine" text
);
SELECT COUNT("interregnum_ended") FROM "list_of_imperial_vicars_1437_1792" WHERE "duration"='3 months, 6 days';