There are three conditions for how many times a hero can use an item during the game.
Once Per Game: A hero can only use the item once during the game (typically during the Battle Phase). Once the item is used, remove it from the game. It cannot be purchased or used again.
Once Per Room: A hero can only use the item once every Battle Phase. Once the item is used, turn it face down. Refresh the item (turn it face up) during the next Setup Phase.
Permanent: Once acquired, the item is always in play and does not require a specific action to take effect. If the item has a shot sequence, a player can choose to perform that sequence instead of the shot sequence on the hero’s Rules card.