When should I have an NPC actively investigate an illusion created by the Phantasmal Force spell?

The phantasmal force spell can be broken if the victim examines the illusion and makes a successful investigation check against the spell’s DC.

What general guidelines should I use to decide when the victim of phantasmal force should use its action to examine the illusion?

Suppose the caster makes an orc think it is being attacked by a wolf.

Should the orc always examine the wolf until the spell ends? Or should the orc use its action to attack the wolf, defend itself, or run away?