If by "acceptable" you mean indicative of a fault with your line I doubt there is a definitive answer. My noise margin varies by up to 6db through the day, though since BT Wholesale provide target margins up to 15db and 6db is normally regarded as acceptable I assume variations through the day of 9-10db are not uncommon. I believe noise is also more prevalent on long lines.
The main thing is to eliminate causes of noise from within your home and within your own wiring. There are help guides within F9 Help & Support to this end. Kitz also provides information on low noise margins here.