Random motion interesting problem

1

I followed the random motion tutorial:
http://www.kirupa.com/developer/actionscript/random_motion.htm

How do I make this play for 3-4 seconds and then delete the movieclip all together?

This is my mc code:

onClipEvent (load) { 
//data you may want to change 
width = 500; 
height = 350; 
speed = Math.round(Math.random()*7)+1; 
//initial positions 
x = this._x=Math.random()*width; 
y = this._y=Math.random()*height; 
x_new = Math.random()*width; 
y_new = Math.random()*height; 
} 
onClipEvent (enterFrame) { 

//x movement 
if (x_new>this._x) { 
sign_x = 1; 
} else { 
sign_x = -1; 
} 
dx = Math.abs(x_new-this._x); 
if ((dx>speed) || (dx<-speed)) { 
this._x += sign_x*speed; 
} else { 
x_new = Math.random()*width; 
} 
//y movement 
if (y_new>this._y) { 
sign_y = 1; 
} else { 
sign_y = -1; 
} 
dy = Math.abs(y_new-this._y); 
if ((dy>speed) || (dy<-speed)) { 
this._y += sign_y*speed; 
} else { 
y_new = Math.random()*height; 
} 
}

And this is the code on the frame:

i = 0; 
while (i<15) { 
//duplicateMovieClip(dot, "dot"+i, i); 
dot.duplicateMovieClip("dot"+i, i); 
i++; 
}