When you start to attack one, time how long it takes to kill it, do that maybe 5 times. Make sure you write down each individual time. Add up all the times when you're finished, then take the sum and divide it by 5. Take that number and divide it by 30, the answer is how many you would kill (on average) in 30 minutes, you can divide it by 60, 120, 180, etc. etc. Hope I helped. :)