That is a bit unusual, IMO.
There was a time we took the monorail from Epcot to MK mid-day, and caught the CM shift change. The Epcot monorail was on the platform, but didn't leave for some 20minutes. At the TTC, we were diverted to the resort monorail. It waited 10min at the TTC before leaving, plus 10min at Poly, plus 5-7 min at GF. It was awful. It was good we didn't have a lunch reservation that day.
There were also multiple times we went to Epcot as a large group and left as Illuminations ended. Part of the group drove back to CR, and part of the group took WDW transportation. Driving took about 15 minutes. WDW transportation took another 90 minutes. BUT one of those two visits the Epcot monorail was down, so WDW only had buses. It was a mess getting on the bus. These were spring trips, so WDW was rather crowded.
We tend to drive to Epcot from CR, but if both monorails are running, it should be faster than 90 minutes. Leaving just as fireworks end adds some time esp if Epcot's busy.