How many years has the winner been a member of the National Football League?
CREATE TABLE "list_of_winners" (
  "year" real,
  "sportsperson" text,
  "nationality" text,
  "team" text,
  "competition_federation_or_league" text,
  "sport" text
);
SELECT AVG("year") FROM "list_of_winners" WHERE "competition_federation_or_league"='national football league';