We have done park open to park close - 8am to midnight! That's sixteen hours! One trip we came pretty close to keeping that up for three days solid (16hrs, then 15 on two subsequent days), On the fourth and fifth days we slept in until about 10, took an afternoon swim break, and then went again until midnight. During vacation times, MK is often open until midnight, so we'd hit one park, have dinner, then head over to MK to close it.
It does help if you have at least one sit down meal per day, and at least take a short sit down break to eat our quck service meals (When we eat around WS, we sometimes forget to sit down, so that gets a little more brutal.) park hopping also affords a short break, assuming you get a seat on the bus.
On other trips we have only managed 14 hour days, including time for breakfast (So I'm counting going to Chef Mickey's for 10 am as starting the day at 10am). One trip we managed to get a fastpass that was timed at 1:30am, I'm not sure about that one, I think it was EMH hours. We didn't need it to get on the ride, so I saved it.
Incidentally, we do this with family in tow. Sometimes the goup had been as small as three adults, but more often it has been a group of eight or nine, with at least three kids. My mom can't always go that many hours, but she usually does pretty well keeping up. She sits out the bigger rollercoasters, so that gives her extra sit down time.
Our family gets a real rush walking into the MK at around 8:30 when lots of other people are leaving. We're the ones skipping down Main Street headed towards BTMRR. We're all real night owls, so it's just the mornings that are a little rough. WE do sometimes take afternoon breaks, but they usually consist of going for a 45minute swim, then hopping in the shower, then heading to dinner and back to a park. If there are young ones along, they may take a nap during this two hour stretch (bus-pool-shower-bus), but not always.
Another critical stategy we employ after a long day, is to take a dip in the hot tub. If ever we get back before 1am, we most certainly try to soak our legs. At swim time, the kids hop in the pool while the adults squeeze in alittle hot tub soaking.
On our last trip, we didn't quite make it until the 3am MK close on our last night. We quit around 1:45, the adults could have kept going, but the kids were done at that point. Usually on the last day in particular, the adreniline kicks in, and everyone wants to hit everything one last time.
So to answer your question, we rarely hit that point at WDW, except that we aren't always able to be at the park gate at 9am. After a full week (Sometimes we only go for five days), we are pretty tired though.