Delete all rows with timestamp older than x days
DELETE FROM on_search
WHERE search_date < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 180 DAY))
DELETE FROM on_search WHERE search_date < NOW() - INTERVAL N DAY
Replace N with your day count
DELETE FROM on_search
WHERE search_date < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 180 DAY))
DELETE FROM on_search WHERE search_date < NOW() - INTERVAL N DAY
Replace N with your day count