It can be done many ways, but you could make a "delay/timer" in your update event and on each frame, decrement this delay. Once it reaches zero, you change the boolean which the box uses to check if it should show or not.

You shouldn't place "logic" inside the OnGUI event as its often rendered more than once per frame. Thats why I added the Update() function here too.

Instead of the timer, you could also do an Invoke timer or other ways. But to remove the GUI element, just encapsulate it in an IF structure.

If you need multiple messages, you could consider a "state-machine" approach instead with switch(state)

But here is a quick solution to your question, I think. Let me know if it worked for you and if you are happy about it. Please mark my answer as the correct one.

var isIntroShown:bool = true; // boolean
var timeIntroShow:float = 5.0f;

function Update()
{

    if(isIntroShown) 
    {
       timeIntroShow-=Time.deltaTime;
       if (timeIntroShow<0.0f)
       {

           isIntroShown = false;
       }
    }
}

function OnGUI()
{
    if (isIntroShown)
    {
       GUI.Box( Rect(100, 200, 750, 50), "You have crashlanded on a strange planet and need to collect 300 crystals to repair your ship and get home. Happy hunting!");
    }

}