The only drones that have an exact timer are the first set that spawn at 27:00. Every drone set after that spawns exactly 3 minutes after the very last drone of the last set dies. For example, first drones spawn at 27:00 and they are dead at 26:00 then the next drones will spawn at 23:00. So if you want to make a macro pay attention to the chat box and when you see the 6th kill hit a macro that will count down the three minutes inbetween give ~60 sec warning until the next spawn.