We were also on the 12 night last June (Dover to Copenhagen via Iceland and Norway). Truly, the trip of a lifetime. We added 3 days in London at the start and 4 in Copenhagen at the end (on our own, none of that booked through Disney).
As others have said, on that one there were 4 at sea days which, given how active we were on port days, felt just right. Lots of entertainment, plenty to do, the downtime on at sea days was welcome. My biggest recommendation: read up on the ports both here and elsewhere, and figure out where you're comfortable doing things on your own. We booked a private guide in Newcastle, for instance, which allowed us to really do what we wanted (lots of archaeology and history) that wasn't offered as a port adventure. Similarly, we hired a private guide in Reykjavik (from a firm that specializes in naturalist expeditions) which made for one heck of a fun day. We also hired a private guide ASAP for Kirkwall- which allowed us to pack in so much, and he was really fantastic. So read up on your ports, decide what's important to you, see if there's a port adventure for that, and if not, figure out if it's reasonable to do on your own. For us, having more time in port doing what we wanted- as opposed to being stuck on a bus- made all the difference in how 'long' the trip felt.
