I would figure 90 min for meals. Keep in mind this is 3 courses, with the app and dessert included in the package. They will often open up the FP reserved area to general seating (15-20 min maybe?) before the show starts. So, I would not book a F! FP.
You can tell your server you are hoping to make F!, and to rush service a bit, and they will try to accommodate. But, do you want to rush through a meal like that for reserved seating for a show?
I would book a 2nd ADR (meal only) at your preferred time. Then, when the show opens, you'll read here how hard it is to get a seat without a package, and decide then which to keep.
I'm thinking, though, unless Pixar is a much higher priority than F!, I would just cancel the package and get an earlier meal time.