user: ### NOW ANSWER:
Question: How many years has the winner been a member of the National Football League?
Columns: ['Year', 'Sportsperson', 'Nationality', 'Team', 'Competition , federation , or league', 'Sport']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1993.0, 'Gary Sheffield', 'United States', 'San Diego Padres', 'Major League Baseball', 'Baseball']
SQL:

assistant: SELECT AVG("Year") FROM "Table" WHERE "Competition , federation , or league" = 'National Football League';