It basically depends on how long I'm in town. When I am in Florida seasonally for a few months in the winter/early spring, I lose track of how many days I visit each park, other than the fact that I like to do a full day at the MK at LEAST once a week as it's my favorite, and I visit MK roughly twice as often as the others. Also, the MK gets most of my late-night park hops after the park I spent the day at closes, since MK is my fave and they stay open later. Also, on days where I don't do a full day at any park, but spend a few hours at one at night, I'm most likely to pick MK, unless something special is going on at one of the others.
Also, I enjoy Epcot more when I treat it like a two-day park, focusing more on Future World on some days and World Showcase on others. This allows for a more thorough exploration of each section without rushing through the self-guided exhibits and, in the case of WS, art galleries and live entertainment.
So, if I was in town for 24 days worth of full-day park going (meaning I was in town for longer than 24 days, because I wouldn't do 24 full days in a row!
, and I actually pre-planned my park ratio, it would look something like this:
MK - 6 Days
Epcot (Future World in it's entirety, with some WS) - 3 days
Epcot (World Showcase in its entirety, with some FW) - 3 days
Animal Kingdom - 3 days
Disney's Hollywood Studios - 3 days
Busch Gardens - 3 days
Sea World - 3 days
This ratio is NOT an exact science written in stone, but it seems to basically be my pattern, as I try to visit all the parks that I have Annual Passes to equally, while giving MK and Epcot double emphasis over the others.
I find that optimally, having at least a 3 day format per park seems perfect for me, as I can go at a very relaxing pace and not feel the need to cram everything into each day. So, on my second day at each park, I make sure to do everything at that park that I missed on the first day, while still having time to repeat faves from the first day. Then, on the third day, if it's the "goodbye" day for that park for that winter/spring, I focus on my favorites and if something/s needs to be cut due to time restrictions, anything that is my least favorite attraction/s at that park is what gets cut.
With 3 days per park, I'd get to visit EVERY attraction in each park at least once, most at least twice, and my absolute favorites on all three days (some more than once per day).
Likewise, if I had 16 total park days, this would approximately translate to a solid 2 days per park (with Epcot counted as two parks) with 4 at MK.
And 8 park days would mean 2 at MK, just 1 day at each other park, with 1 day each for the 2 halves of Epcot.
That's about as short as I would want to go for an out-of-season trip, unless there was something REALLY SPECIAL going on at one of the parks that would prompt me to drive to Florida for a short weekend trip (such as a beloved attraction closure, or Summer Nightastic/Star Wars weekends, which I did last summer for 3 days just in case the MSEP didn't last through the following winter). In those cases I don't worry about having enough days to visit each park once, because such a trip would be an "extra" trip motivated solely by a special reason. So I'd just focus primarily on whatever park/s had the special events going on that prompted the trip.