How many times was Jillian Grace the centerfold model?
CREATE TABLE "table1_1566852_6" (
  "date" text,
  "cover_model" text,
  "centerfold_model" text,
  "interview_subject" text,
  "20_questions" text,
  "pictorials" text
);
SELECT COUNT("date") FROM "table1_1566852_6" WHERE "centerfold_model"='Jillian Grace';