Usually 12 ish BUT that is when the coaches arrive hence independant travellers are usually told 2 so that they do not have to queue for ages. You usually have to board by 4pm latest. We usually aim for 3pm and are aboard and in our cabin in 15mins tops ! Have a lovely cruise (but dont be late).
Check in usually opens at around 12.15 (last two times on Arcadia) presume all PO ships are same, even then there is often a queue but quite fast moving, we always get there early as it is usually a 4 hour drive and you never know about traffic conditions.