When were 668 units produced?
CREATE TABLE table_17690 (
    "Quattroporte IV" text,
    "Units Produced" real,
    "Production Period" text,
    "Engine Capacity" text,
    "Power" text,
    "Max Speed" text
)
SELECT "Production Period" FROM table_17690 WHERE "Units Produced" = '668'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: When were 668 units produced? ### Input: CREATE TABLE table_17690 (
    "Quattroporte IV" text,
    "Units Produced" real,
    "Production Period" text,
    "Engine Capacity" text,
    "Power" text,
    "Max Speed" text
) ### Response: SELECT "Production Period" FROM table_17690 WHERE "Units Produced" = '668'