You'll probably get two replies...
"Not bad at all"
and
"Awful"
Unfortunately, the travel time will depend on too many things... what time of year, what time of day, how busy they are, how much patience the guests have, etc. Look at the following scenarios...
Family 1
Arrives at bus stop at 8:05. Bus shows up at 8:10. They're the last stop at the resort and go straight to EC. They're walking through the gates at 8:30.
Family 2 (same day)
Arrives at bus stop at 8:12. Watch a MK bus come and go... then a DHS bus, then an MK bus, then AK. A group shows up at the stop with someone in a wheelchair. 8:35 EC bus pulls up, the wheelchair gets loaded and the group takes the rest of the remaining spots. Next bus to EC shows up at 9:00. They're doing the stops in reverse for some reason and the family gets to EC at 9:45.
Obviously Family 1 will say the transportation was great and Family 2 will say it was terrible. The truth is somewhere in the middle.
The 'general' rule to follow is allow an hour to get from point A to point B using Disney transportation.