Last trip we checked into Sci-Fi 15mins early for our midday ADR as suggested and were told the wait was 45-60 minutes. Ugghh OK, we figured out something to do in the area for a bit and returned 40 minutes later at the podium. There was still a wait so we stay there this time. Shouldn't be much longer, right? No, we wait almost another hour to be seated. 90 minutes total. We were happy once we finally got some food in our stomachs. Oddly during the entire 60 minutes of our slow service dining, there were always one-third of the tables empty. Tables were cleaned but remain empty for 20-30 minutes before they were re-seated. Why?
^
tl;dr- 90min wait at Sci-fi.
Not my usual experience with ADRs but several times we've had seriously long waits. Twice we have messed up our ADRs and arrived at the wrong time and they still honored us

...a bit of give and take.
As long as you don't abuse ADRs on a regular basis, i don't feel there's anything "wrong" with finessing an important one if the need arises.
I think the best plan would be as a previous poster suggested: Stop there before you go to F! and let them know your position. If they can accommodate your need, great. If not, you'll need to exit F! a few minutes early.
What i have issue with is people who book a ton of ADRs knowing they'll cancel more than half of them the week/day before. That deserves a good smack