user: Question: When were 668 units produced?
Columns: ['Quattroporte IV', 'Units Produced', 'Production Period', 'Engine Capacity', 'Power', 'Max Speed']
Types: ['text', 'real', 'text', 'text', 'text', 'text']
Sample row: ['2.0i V6 24v', 587.0, '1994–1998', '1,996 cc', '287PS (211kW) @ 6500rpm', '255km/h (158mph)']
SQL:

assistant: SELECT "Production Period" FROM "Table" WHERE "Units Produced" = 668.0;