This item is very rare. It appears a handful of times in Serious Sam: The Second Encounter, and only once in both Serious Sam: Next Encounter and Serious Sam: Double D. Even in The Second Encounter, Invulnerability can be found on the main path once. The rest of them are hidden in secrets.
Invulnerability is extremely useful, so the player should pick it up as soon as they can or the hardest parts in longer battles.
Invulnerability lets the player use strategies that would otherwise be too dangerous to use, such as running to the middle of a group and spamming Rockets and grenades at point-blank range. Take advantage of this to kill enemies faster and easier that normal.
When Invulnerability is almost over, the player should start maneuvering themselves so that they'll be in a safe place when it runs out. Otherwise, they may be in a position where they'll take a lot of damage when Invulnerability runs out.