Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
How many years has the winner been a member of the National Football League?

### Inputs:
table_name: table, columns: ['Year', 'Sportsperson', 'Nationality', 'Team', 'Competition , federation , or league', 'Sport']

### Response:
SELECT AVG Year FROM table WHERE Competition , federation , or league = national football league