CREATE TABLE table_74920 ("Race" TEXT, "Class ( es ) " TEXT, "Length/Duration" TEXT, "Circuit" TEXT, "Date" TEXT)

What was the date of the race that lasted 6 hours?
SELECT "Date" FROM table_74920 WHERE "Length/Duration" = '6 hours'
SELECT "Date" FROM "table_74920" WHERE "Length/Duration" = '6 hours'