How to control interval between twice touch?


I wanted to do some change with the SimpleGame project in the examples of ios
I spped up the monster, make them more
But when I touch the screen fast, the bullet will go everywhere and monster will never close with me
So, how can I control the interval between twice touch?
For example, I touch screen once, spirit shoot, then I have to wait for 0.2s to second shoot
In the 0.2s, I touch, it doesn’t work.
My Endlish is so poor, thanks for your help.