This espionage must be carried out every time before an attack.
But not if the target is:
- A Nomad Camp, in Nomad Event .
- A Samurai Camp, in Samurai Event .
- A BerimondTower in Berimond Event .
This is because, if espionage is carried out once, for a target, of the above, then every target of the same level has exactly the same defense plan. For example, every Nomad Camp in the same map has the same defense plan and every Nomad Camp level 22 has the same defense plan.
You can see spy reports for every target of the above categories here: