My original script is the following: private var timeSinceLastCollision = 0;
so where do I put the script you wrote and what do I put in the spot where you wrote (collecteditem)
The Robot Guards of the 3D Platformer Tutorial on the Unity website respawn when the player can no longer see them. They are Instantiated from a prefab on this line:
Have a look at the tutorial I've linked to above and instantiating prefabs in the Unity Manual.
To respawn the item after a certain amount of time, you could start a timer once the player collects the object. Once the timer reaches the time limit you have specified, you can then respawn the object. Something like the following in your Update function: