The single most important thing it depends on is conditions on the ground at the particular moment you happen to go to the attraction.
I would agree with that being a very big thing, along with just what accommodations the person needs.
We just it back from WDW 2 weeks ago tomorrow.
These 3 situations happened on the same day at Small World. That attraction has a handicapped line that is used by people with mobility devices and also with GACs for a quieter, less crowded place to wait.
1) The posted wait for the regular line was 20 minutes. The handicapped line was all the way up the ramp past the gate to that area. We choose not to go because, in past experience, that would equal a 30-45 minute wait, depending on how many in the line needed the wheelchair boat.
2) About 1 hour later, the posted wait in the regular line was still 20 minutes. We got into the handicapped line and had one group ahead of us. Our wait was about 10 minutes.
We did wait for the wheelchair boat, but only one group was taken from behind us to board a regular boat - and they ply to on 2 boats ahead of us.
3) By the time we got off, the handicapped line stretched almost all the way to the top of the ramp. So, about a 20-25 minute wait, when the regular line was posted as 20.
So, how long you wait depends a lot (at least in our experience) on what is going on at the attraction. By he end of a trip, it does tend to even out, with waits or one attractions shorter and some longer than the average guest.