Dinners typically don't go that long, but you never know.
PLENTY of our dinners have gone that long!!!!
PLENTY of times we've been seated WELL PAST our ADR time. It REALLY depends on many variables: party size, time you are dining, popularity of location.
We can reliably predict we'll be seated late at 'Ohana and Boma dinners, especially when we opt for the 6:30pm - 7:30pm time frame, even on slow weeks. Sometimes we've been seated a full hour past our ADR.
Sometimes we go to WDW in small groups, sometimes large groups. All have been seated well past ADR times, but it is especially true during free dining promo and when our party is large. With a group of 10+, hour waits are sadly not uncommon. Places that aren't full will typically do better.(lunch, places like GF Café, early time slots: 7am breakfasts or 4pm dinners)
The BIGGEST unknown though is transportation. If your ADR and FP are together- like the WS paired with TT, transportation isn't likely to be an issue. Busing (resort to park) commonly takes an hour- ONE way after dinner. Getting to DTD also takes a long time.
These days, like all this summer, the monorails are EXPECTED to be down. If running, getting from Poly/GF to MK is quick via monorail. Via bus, it's another beast entirely. The monorail is going to be down quite a bit this summer!
The drive- just the bus drive, no waiting - between Contemporary/MK/Poly and DTD (before the construction) takes a full half hour via bus (with traffic lights and water park stop).
Typically, once seated, single credit meals take 90 minutes, signature meals reliably take more like 120minutes (just eating), but it also depends how many courses. Seeing ALL the characters at character meals sometimes also means allowing extra time. If you just want to eat and go, and don't want dessert/appetizers, some meals can be finished in an hour.
Oh, if you plan a dinner show, like Hoop de Doo, just relax and allow the whole evening for it.
Coming for your resort, via WDW transport, timing is going to vary widely.
Our ADR's tend to be dinner, and we tend to finish our 3 FP before dinner, but even some of us experienced folks run into mishaps!
Now Fastpass usage: most only take 20min, but some take more like an hour...maybe much longer if you add commuting time.