We did 8 days last year, and we're going back this year for another 8! For us, that amount of time is just perfect.
It was brilliant - we were traveling with our 3 year old, and were able to slow down and take things at his pace. The end result was ZERO meltdowns, and nothing but an amazing trip!

I was initially worried that it was too much time, but it was just perfect - we didn't have to rush because we knew we had the time, and were also able to do the things we loved over and over again. It also gave us a chance to try lots of different restaurants, and take the time to enjoy all the shows, parades, etc. that sometimes have to be missed when you have less time.
We felt satisfied with our time at the end of the trip - but even after the 8 nights, were more than ready to go back again just a few weeks later!

Sadly, that wasn't an option, but that's why we're already making plans for our trip in Nov/Dec this year.