the only way i thought off (but no time for test
) is this:
adding a small ufo befor the normal wave make this ufo appear only a (very) short amount of time
then add a huge wait time between the next wave like 3 months (in seconds?)
so you can be assured that this mission main ufos only appear after X time BUT if this mission is selected in the early months there will be less ufo to shut down during that time
also if the list of possible mission gets smaller until mission reset and the last mission generated is such a "waiting"-mission you will have a huge (time X) waiting period where nothing is happening until mission reset