// at the top:
private long hitStart, hitTime = 2000; // 2000 millis = 2 secs

// change your method:
public void jumpBackWhenHit(int jumpHeight){  
        // bepaal richting  
        // jump back when hit  
        Actor actor = getOneIntersectingObject(RibbonPig.class);  
  
        if(actor != null){  
            System.out.println("I've been hit");  
            // get back a little in x-direction  
            flickeringRogue();  
            jump(jumpHeight);
            hitStart = System.currentTimeMillis(); // now
        }  
        // be untouchable for 2 seconds  
    }

// where you check the touches:
if (hitStart +hitTime < System.currentTimeMillis()) { // now you're not untouchable anymore.