DeletedUser27062
Guest
Six is the max that will ever be on the screen at a time, but every time an hour, or however long the interval really is, passes, one of the old spawns will disappear and be replaced by a new one. I have watched it happen more than once in previous events. I am about to click on a spawn, the message about a new spawn appears on the screen, and the one I was about to click on has disappeared and been replaced by a new one in a different location.
I've seen it too and noted it in a city I was monitoring last night. At first I assumed I had accidently collected a drop but then noticed it again on another world. I allowed a city to become full with 6 drops and took screenshots of the donkeys, checked back hourly to see if they disappeared and sure enough they would just as you described so EC does expire.