You can use Input.GetButton("buttonName"): this function returns true while the button is pressed. There are equivalents for keys (Input.GetKey("a"), for instance) and for GUI ( GUI.RepeatButton)

EDITED: Well, doing what you want with GUI buttons is more complicated than I supposed: contrary to what I thought, RepeatButton doesn't return true while pressed - it actually returns true and false alternately each OnGUI! This complicates things, because there's no steady state to rely on.
But the show can go on: since Unity doesn't return a steady state for us, let's create our own! We can use a boolean flag, speedButton, which will be set when RepeatButton returns true and reset when the mouse button is released.
To simplify things a little, let's check the time only when RepeatButton is released: if it's greater than the limit, select high speed, otherwise select low speed:

var speed: float;
var timeThreshold: float = 0.6; // 600mS to select high speed

private var speedButton = false; // current button state
private var lastState = false; // last button state
private var speedTime: float = 0; // time measurement

function OnGUI(){
  // create a steady state result for RepeatButton:
  // set speedButton when RepeatButton is true...
  if (GUI.RepeatButton(Rect(10,10,200,100), "Speed")){
    speedButton = true; 
  }
  // and reset it when mouse button released:
  speedButton &= Input.GetMouseButton(0);
  // now speedButton can be verified:
  if (speedButton != lastState){ // if button changed state...
    lastState = speedButton; // register current state
    if (speedButton){ 
      // if button down define the threshold time
      speedTime = Time.time + timeThreshold;
    } else {
      // if button up select speed according to time elapsed
      speed = (Time.time > speedTime)? 10 : 5;
    }
  }
}