I wouldn't even consider just one day! I don't like to, but I have settled for 1.5 days... There is just so much to do, and the repeatability of attractions makes it even better. I could probably spend a whole week there. Come to think of it, that is part of why I want to work the Disney College Program.
We usually go for 10-11 days, and 3 of those days would always be to MK. This year, we only stayed for 8 nights, which gave us 7 1/2 days. We were able to spend 2 1/2 of those days at MK. My sister and I are going in January for our shortest trip ever (6 nights), and I can't see us spending any less than 2 days at MK.
I very seldom spend a whole day in any given park...usually get to a park in the morning at opening and leave around 2ish. Or go to a park late afternoon and spend the evening there. So, I go to MK about 3 times each visit. I love it in MK after dark, so it works out that I'm there twice during the day and once a night, usually towards the end of my trip.
I cannot even think of going tp WDW without spending AT LEAST 2 days at MK. There is so much to see and do for all groups of ages, it is really the best park.
Lets see... looking at my schedule for my upcoming 7 day trip I have MNSSHP 7-midnight on the 16th and 8:05 breakfast at the Crystal Palace the morning of the 20th with plans to leave around 2pm that day. Does that count as one day? 3/4 of a day?