user: Question: What is the length of the First Union Six Hours at the Glen?
Columns: ['Race', 'Length', 'Class', 'Circuit', 'Date']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Rolex 24 at Daytona', '24 Hours', 'All', 'Daytona International Speedway', 'February 3 February 4']
SQL:

assistant: SELECT "Length" FROM "Table" WHERE "Race" = 'First Union Six Hours at the Glen';