I also thought that rotary times were very long until I timed how long it takes for a full rotation of the head. This can vary from head to head so I would suggest going out to your rotary head with the zone running (made easy with the iro) and see how long it takes for the head to make a full pass. Once you have that, it might make sense why the time is 24 minutes. It could only be 10 rotations or something like that which would be with in reason.
As for spray heads they cover less area with similar a gpm. There for don't require as much time.
A quick look at the precipitation per hour of a Hunter rotary and spray head show that the suggested times are pretty close.
Spray head = 3.89 inches precipitation per hour
Rotary head = 0.81 inches precipitation per hour
So they multiplier would be 4.8 more water applied every hour with a spray head. So in this example you ran the spray head for 7 minutes you would have to run the rotary for 34 minutes for the same coverage.
I hope that helps. Now with your iro you can make adjustments when every you what as you see fit.